If you are raped, there are certain things you have to do immediately to increase the chances that your attacker will be apprehended and hopefully put away.
Do not immediately bathe, wash or dispose of the clothes you wore during the attack--your body and clothes can provide important evidence for catching and prosecuting the rapist.
Tell someone--the police, a doctor, a friend or a counselor at a rape crisis center. There are rape crisis centers in every city, at most hospitals and on most college campuses. The national rape crisis phone number is 800-656-HOPE, and they will refer you to the closest center in your area. It might feel strange to contact some center for help where you don't really know the people who will be helping you. However, the beauty of a rape crisis center is that the staff and volunteers are trained to deal with these situations. They don't need personal information from you. They are there to help you.
As soon as possible, get yourself to a hospital or clinic. There you will receive the standard rape kit procedure, which collects evidence of rape. You will be able to talk to a counselor and will be told where you can get some follow-up counseling, which is highly recommended. Also, you will be offered emergency contraception and antibiotics to fight potential STDs.
In the weeks and months following a rape, get tested for STDs, pregnancy and, about six months after the attack, HIV infection.
Consider continuing counseling. After going through counseling, eventually it may be useful to join a rape support group. It is common to feel some sense of alienation from people who have never been through the sort of nightmare that you have endured after a rape, so meeting with a group of other rape survivors could be useful.