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OK, you have decided to become 7+ feet of implacable, vicious, deadly, murderous, fanatical, sociopathic, and highly trained engine of death and destruction which is tasked with defending the human race (with the definition of human being extremely rigid) in the name of a rotting demi-god sitting on a golden throne that no one has spoken to for about 10 thousand years.  OK, you do you.  But how do you go about this?  What gets your foot in the door to become this badass of lethality?  Is there an office or someplace to apply?  Do you have to do something incredible to get noticed?  Well that depends on which Astartes Chapter you want to join, which sorta depends on where you live.  If you live in the Ultramar sub-sector, you can apply to the Ultramarines and it is just like the normal army to a point.  Go to any church or temple dedicated to the Emperor of Man and you can join the Black Templars (who also kidnap talents from worlds they are currently campaigning on).  If you live on Fenris (home of the famous Space Wolves), you have to become mortally wounded in an epic battle in which you barely survive being incredibly outnumbered or have faced insurmountable odds.  Or if you live on a Death or Feral world, surviving battles or just making it to being a pre-teen is enough.  Also, some of the chapters recruit by hunting down and capturing criminals or gang members to be tested to see if they would make great Astartes, so each chapter may have their own way of recruiting.  The chapters value a strong killer instinct and aggression, especially if it is near the level of being psychotic.

Oh yeah, if you are over 14 years old you are too old to attempt to join the Space Marines. And you must be male, sorry ladies, the Adeptus Sorritas recruiting station is just over there at the local place of worship.  This is because the genetic upgrades and implanted organs (of which there are 22) only work on males.  After being kidnapped, recruited, or rescued, the applicant is tested for genetic purity :no mutants or xenos need apply because if you test as one of these you will be killed immediately, probably with fire.  Also applicants are tested for bio-compatibility and are given psychological tests to see if they meet the standards of being a killing machine, but these standards are that one can think and plan while joyously murdering xenos, cultists, and heretics.   At this point the candidate is knows as an Aspirant, to move to the next phase they must survive the implantation of the new organs and the genetic upgrades, which only small percentage of aspirants can, the rest die horrific deaths, but hey, you want human killing machines, you have to break a few humans.  Oh, and by the way, don’t bring any pics of home or of family members as you are probably going to have your old memories removed and new ones implanted so that your only loyalty is to the chapter and the Emperor.

During the implantation process the candidate becomes known as a neophyte and once they survive the implantation process, learn how to use the organs (and yes, they have some mental control of their organs), and the hypnotherapy they become known as Brothers.

I will cover what a Brother has to endure to become a fully armed and armored space marine later, but for now, here is a breakdown of what the different implanted organs are and do.

Since I do not have the patience to type all of this information – I have copied this from here and added my own comments in italics:

The 19-22 implanted organs are very complicated, and because several of them only work properly or at all in the presence of other implants, the removal, mutation, or failure of one organ can affect the precise functioning of the others.^[1][2a][3] Because of this, and the fact that each Chapter’s gene-seed belongs to that Chapter alone, different Chapters display different characteristics and use different sets of implants and methods of implantation.^{[Needs Citation]} A “geneseed organ” retains this name even when it is spread across multiple implants, such as the Progenoids or Betcher’s Gland, and/or is a cybernetic implant rather than an organ in the traditional sense.

Throughout the implantation process the Marine must undergo various forms of conditioning in order for the implanted organs to develop and become part of his physiology.

Listed below is the complete set of implants used:

• • Phases 1-3 can be introduced at the same time, ideally between 10 and 12 years of age; Phase 1 remains ideal until age 14, but 13-14 are not ideal for Phases 2 or 3.
    • If the Marine is receiving Primaris implants, the Sinew Coils and Magnificat are introduced between Phases 3 and 4, in that order or simultaneously; their ideal age is 12-13. The Belisarean Furnace is presumably introduced during this time as well, but in what order, and at what ideal age, is unknown; apparently all three are always introduced across at most 2 steps, so the Furnace is presumably always simultaneous with at least one of the other two. These 1-2 phases are unnumbered, and are instead known as the Primaris Alpha and Primaris Beta phases.
  • Phases 4 and 5 can be introduced at the same time, ideally between 12 and 14 years of age.
  • Hypnotherapy normally begins at phase 6, ideally sometime between 14 and 17 years of age.
  • Phases 7 to 9 are normally introduced simultaneously, ideally at a point between 14 and 16 years old.
  • Phases 10 and 11 are also ideally implanted between the ages of 14 and 16.
  • Phases 12 and 13 are ideally implanted between the ages of 15 and 16.
  • Phases 14 and 15 may be introduced at the same time, ideally between 15 and 16 years of age.
  • Phases 15 to 19 are then ideally introduced to the recipient between the ages of 16 and 18.^[1][2a][3]

Secondary Heart

Phase 1: This simplest and most self-sufficient of implants allows a Space Marine to survive his other heart being damaged or destroyed, and to survive in low oxygen environments. Not just a back-up, the secondary heart can boost the blood flow around the Marine’s body.^[1][2a][3]

Ossmodula

Phase 2: A small, complex, tubular organ, the ossmodula secretes hormones that both affect the ossification of the skeleton and encourages the forming bone growths to absorb ceramic-based chemicals that are laced into the Marine’s diet.^[1][2a][3] This drastically alters the way a Space Marine’s bones grow and develop. Two years after this implant is first put in the subject’s long bones will have increased in size and strength (along with most other bones), and the rib cage will have been fused into a solid, bulletproof mass of interlocking plates (so hitting them in the chest will do fuck all for the normal human).^[1][2a][3]

Biscopea

Phase 3: This small, circular organ is inserted into the chest cavity and releases hormones that vastly increase muscle growth throughout the marine’s body. It also serves to form the hormonal basis for many of the later implants.^[1][2a][3]

Primaris Implants

• • Main article: Primaris Space Marines

In addition to the above, the new line of Primaris Space Marines are given three additional implants that allow for their superior size and strength in addition to a greater level of genetic stability.^[8] The Primaris Implants take place between Phases 3 and 4, known as the Primaris Alpha and Primaris Beta phases. Both phases can be introduced simultaneously.^[10]

Sinew Coils

Known as the Steel Within, this implant is the only one which is cybernetic. The Space Marne’s sinews (tendons and ligaments) are reinforced with durametallic coil-cables that contract with incredible force, magnifying the subject’s strength beyond that of a regular Space Marine and giving another layer of interior defense.^[9]

Magnificat

Known as the Amplifier, this small thumbnail-sized lobe is inserted into the brain’s core. The implant secretes hormones that increases the body’s growth functions while also intensifying its advanced systems, especially for the ossmodula and biscopea. In truth, this implant is but half of the true, dual-valve immmortis gland (the “God-Maker”) which the Emperor made for the Primarchs. Belisarius Cawl was able to build the dextrophic lobe (right half) but discovered that information on the sintarius (left half) had been wholly eradicated by an unknown force.^[9]

Belisarian Furnace

Known as the Revitalizer, this dormant organ connects to both hearts. In times of extreme stress or trauma, it expels self-manufactured chemicals similar to combat stimms that also aid in regeneration. After activation, the gland will fall dormant again, taking some time to build itself up for activation once more.^[9]

Haemastamen

Phase 4: Implanted into the main circulatory system, this tiny implant not only increases the haemoglobin content of the subject’s blood, making it more efficient at carrying oxygen around the body and making the subject’s blood a bright red, it also serves to monitor and control the actions of the phase 2 and phase 3 implants.^[1][2a][3]

Larraman’s Organ

Phase 5: A liver-shaped organ about the size of a golf-ball, this implant is placed within the chest cavity and connected to the circulatory system. It generates and controls ‘Larraman cells’, which are released into the blood stream if the recipient is wounded. They attach themselves to leukocytes (white blood cells) in the blood and are carried to the site of the wound, whereupon contact with air they form a near instant patch of scar tissue, sealing any wounds the Space Marine may suffer – acting like platelets, only better.^[1][2a][3]

Catalepsean Node

Phase 6: Implanted into the back of the brain, this pea-sized organ influences the circadian rhythms of sleep and the body’s response to sleep deprivation. If deprived of sleep, the catalepsean node cuts in. The node allows a Marine to sleep and remain awake at the same time by switching off areas of his brain sequentially. This process cannot replace sleep entirely, but increases the Marine’s survivability by allowing perception of the environment while resting^[1][2a][3]. This means that a Space Marine needs no more than 4 hours of sleep a day, and can potentially go for 2 weeks without any sleep at all (fuck this, I really enjoy my sleep).^[1][2a][3]

Preomnor

Phase 7: This is essentially a pre-stomach that can neutralise otherwise poisonous or indigestible foods. No actual digestion takes place in the preomnor, as it acts as a decontamination chamber placed before the natural stomach in the body’s system and can be isolated from the rest of the digestive tract in order to contain particularly troublesome intake.^[1][2a][3]

Omophagea

Phase 8: This implant, also called “the Remembrancer”, allows a Space Marine to ‘learn by eating’. It is situated in the spinal cord but is actually part of the brain. Four nerve bundles are implanted connecting the spine and the stomach wall. Able to ‘read’ or absorb genetic material consumed by the marine, the omophagea transmits the gained information to the Marine’s brain as a set of memories or experiences. It is the presence of this organ which has led to the various flesh-eating and blood-drinking rituals for which the Astartes are famous, as well as giving names to chapters such as the Blood Drinkers and Flesh Tearers. Over time, mutations in this implant have given some chapters unnatural craving for blood or flesh.^{[1][2a][3][6]}

Multi-lung

Phase 9: This additional lung activates when a Space Marine needs to breathe in low-oxygen or poisoned atmospheres, and even water. The natural lungs are closed off by a sphincter muscle associated with the multi-lung and the implanted organ takes over breathing operations. It has highly efficient toxin dispersal systems (so no more bong hits – boring).^[1][2a][3]

Occulobe

Phase 10: This implant sits at the base of the brain, and provides hormonal and genetic stimuli which enable a Marine’s eyes to respond to optic-therapy. This in turn allows the Apothecaries to make adjustments to the growth patterns of the eye and the light-receptive retinal cells – the result being that Space Marines have far superior vision to normal humans, and can see in low-light conditions almost as well as in daylight.^[1][2a][3]

Lyman’s Ear

Phase 11: Not only does this implant make a Space Marine immune to dizziness or motion sickness, it also allows Space Marines to consciously filter out and enhance certain sounds. The Lyman’s Ear completely replaces a Marine’s original ears. It is externally indistinguishable from a normal human ear (I thought men already had this as I can filter out my wife’s voice extremely well).^[1][2a][3]

Sus-an Membrane

Phase 12: Initially implanted above the brain, this membrane eventually merges with the recipient’s entire brain. Ineffective without follow-up chemical therapy and training, with sufficient training a Space Marine can use this implant to enter a state of suspended animation, consciously or as an automatic reaction to extreme trauma, keeping the Marine alive for years, even if he has suffered otherwise mortal wounds. Only the appropriate chemical therapy or auto-suggestion can revive a Marine from this state. The longest recorded period spent in suspended animation was undertaken by Brother Silas Err of the Dark Angels, who was revived after 567 years^[1][2a][3] (d.321.M37)^[11]

This organ is also known as the hibernator implant.^[13]

Melanchromic Organ

Phase 13: This implant controls the amount of melanin in a Marine’s skin. Exposure to high levels of sunlight will result in the Marine’s skin darkening to compensate. It also protects the Marine from other forms of radiation.^[1][2a][3]

Oolitic Kidney

Phase 14: In conjunction with the secondary heart this implant allows a Space Marine to filter his blood very quickly, rendering him immune to most poisons. This action comes at a price, however, as this emergency detoxification usually renders the Marine unconscious while his blood is circulated at high speed. The organ’s everyday function is to monitor the entire circulatory system and allow other organs to function effectively.^[1][2a][3]

Neuroglottis

Phase 15: This enhances a Space Marine’s sense of taste to such a high degree that he can identify many common chemicals by taste alone. A Marine can even track a target by taste.^[1][2a][3]

Mucranoid

Phase 16: This implant allows a Space Marine to sweat a substance that coats the skin and offers resistance to extreme heat and cold and can even provide some protection for the marine in a vacuum. This can only be activated by outside treatment, and is common when Space Marines are expected to be fighting in vacuum.^[1][2a][3]

Betcher’s Gland

Phase 17: Consists of two identical glands, implanted either into the lower lip, alongside the salivary glands, or into the hard palette. The gland works in a similar way to the poison gland of venomous reptiles by synthesizing and storing deadly poison, which the Marines themselves are immune to. This allows a Space Marine to spit a blinding contact poison. The poison is also corrosive and can even burn away “strong” metals, given sufficient time.^[1][2a][3]

Progenoids

Phase 18: There are two of these glands, one situated in the neck and the other within the chest cavity. These glands are vitally important and represent the future of the Chapter, as the only way new gene-seed can be produced is by reproducing it within the bodies of the Marines themselves. This is the implant’s only purpose. The glands absorb genetic material from the other implanted organs. When they have matured, each gland will have developed a single gene-seed corresponding to each of the zygotes which have been implanted into the Marine.^[1][2a][3]

These take time (5 years in the first case, 10 in the latter)^[1][2a][3] to mature into gene-seed. The gene-seed can then be extracted and used to create more Space Marines. Thanks to the superior implantation process and genetic stability of Primaris Space Marines, the Imperium has recently been able to harvest gene-seed at a rate previously unseen.^[8]

Black Carapace

Phase 19: The most distinctive implant, it resembles a film of black plastic that is implanted directly beneath the skin of the Marine’s torso in sheets. It hardens on the outside and sends invasive neural bundles into the Marine’s body. After the organ has matured the recipient is then fitted with neural sensors and interface points cut into the carapace’s surface.^[1][2a][3] This allows a Space Marine to interface directly with his Power Armour. Without the Black Carapace many of the systems of the power armour will not function. While driving the vehicles of the Chapter, a special spinal interface plugged into the power armour and Black Carapace provides the Space Marine an intuitive ‘feel’ for vehicles systems and controls, literally making him a part of his vehicle.^[7] A similar cybernetic used by the Imperium of Man is the Mind Impulse Unit.

The Black Carapace was originally developed during the Unification era by the Terran scientist Amar Astarte. However, it was flawed, and could not be utilized until Ezekiel Sedayne perfected the technology.^[12]

Conditioning

• • Chemical Treatment – Until his initiation, a Marine must submit to constant tests and examinations. The newly implanted organs must be monitored very carefully, imbalances corrected, and any sign of maldevelopment treated. This chemical treatment is reduced after completion of the initiation process, but it never ends. Marines undergo periodic treatment for the rest of their lives in order to maintain a stable metabolism. Marine power armour suits contain monitoring equipment and drug dispensers to aid in this.^{[Needs Citation]}

• • Hypnotherapy – As the super-enhanced body grows, the recipient must learn how to use his new abilities. Some of the implants, specifically the phase 6 and 10 implants, can only function once correct hypnotherapy has been administered. Hypnotherapy is not always as effective as chemical treatment, but it can have substantial results. If a Marine can be taught how to control his own metabolism, his dependence on drugs is lessened. The process is undertaken in a machine called a Hypnomat. Marines are placed in a state of hypnosis and subjected to visual and aural stimuli in order to awaken their minds to their unconscious metabolic processes.^{[Needs Citation]}

• • Training – Physical training stimulates the implants and allows them to be tested for effectiveness.^{[Needs Citation]}

• • Indoctrination – Just as their bodies receive 19 separate implants, so their minds are altered to release the latent powers within. These mental powers are, if anything, more extraordinary than even the physical powers described above. For example, a Marine can control his senses and nervous system to a remarkable degree, and can consequently endure pain that would kill an ordinary man. A Marine can also think and react at lightning speeds. Memory training is an important part of the indoctrination too. Some Marines develop photographic memories. Obviously, Marines vary in intelligence as do other men, and their individual mental abilities vary in degree.^{[Needs Citation]}

After all of these implantations and alterations to the human body, there is a serious debate whether or not Space Marines are human. While they indubitably serve humanity, they are at least two meters tall and can breathe poison and eat through metal.

End of material from Lexicanum.

Oh and by the way, I think the neophytes are awake for a lot of these surgeries, to teach them how to deal with pain and trauma.  Ugh