Before you can have gender reassignment (sex change), you must follow the guidelines outlined in the Harry Benjamin Standards of Care (SOC). Basic requirement are that you must be18 years of age; pass a mental health evaluation; receive documentation letters that recommend surgery; have a one-year real life experience (RLE) during which you dress and live as the target gender; and take hormones for at least 12 months to change your physical appearance.
During this time, if you are transitioning from male to female, you may have electrolysis or laser treatments for hair removal. If mental health professionals recommend further counseling or therapy, you must complete that as well. Generally, physicians won't prescribe hormones (estrogen for males and testosterone for females) until you have lived as your target gender for three months, after which you must take them for a year. Thus, the shortest time before you can have surgery is 15 months, often considerably longer.
Male to Female
Michael Brownstein, MD, of the San Francisco Medical Society discusses different types of gender reassignment surgery.
In a common procedure for male-to-female reassignment surgery, a curved incision is made around your scrotal sac and the testes removed. An incision is made up the length of the penis, the skin removed, and the penile tissue cut away. The surgeon retains the head of your penis to form a clitoris. The surgeon removes your urethra (urine tube) and attaches it below the new clitoris, with a catheter (plastic tube) to keep it open while it heals.
The surgeon makes an opening behind where the testes had been to form a vagina, and the skin that was removed from the penis (and sometimes some scrotal skin) is inverted and sutured into a tube, which is then placed inside the new vaginal opening to create the vagina. Skin from the scrotum forms new labia.
Female to Male
The first steps in female-to-male gender reassignment surgery are removal of the breasts, uterus and ovaries, and closing the vagina. You may have liposuction on the buttocks, hips, and thighs for a more masculine shape. You can choose to use a prosthesis to give the appearance of a penis, or you can have a "micropenis" created (metoidioplasty). The male hormones increase the size of the clitoris, and the clitoris and urethra (or the clitoris alone) can be cut out and lengthened to form a small (4 to 7 cm) penis. If the urethra (urine tube) is included in the penis, you can stand to urinate, but you can't obtain an erection.
The last option, phalloplasty creates a larger penis but requires that skin be cut from a donor site on your body (often your arm) to create a new penis. The skin is rolled in a tube around a plastic catheter so a new urethra will form. The surgeon sutures the penis into place, attaching to the nerves of the clitoris to provide some sensation. In a later stage, you can have a penile prosthesis inserted, usually an inflatable prosthesis, so you can achieve an erection. Your labia forms the scrotum to hold testicular prostheses.
Michael Brownstein, MD, of the San Francisco Medical Society discusses different types of gender reassignment surgery. 
The first steps in female-to-male gender reassignment surgery are removal of the breasts, uterus and ovaries, and closing the vagina. You may have liposuction on the buttocks, hips, and thighs for a more masculine shape. You can choose to use a prosthesis to give the appearance of a penis, or you can have a "micropenis" created (metoidioplasty). The male hormones increase the size of the clitoris, and the clitoris and urethra (or the clitoris alone) can be cut out and lengthened to form a small (4 to 7 cm) penis. If the urethra (urine tube) is included in the penis, you can stand to urinate, but you can't obtain an erection. 
