What do I do if I've been sexually assaulted? I don't want my parents to know.

Get help! This is very sad. You can get help from the er, the police, your doctor, or your ecclesiastical leader. Please reach out to someone, no one should have to deal with that. Consider talking to your parents.
Shamed. Being a victim of sexual assault is not something to be ashamed of. It is not your fault. If you believe your parents are likely to be supportive, then tell them what happened. You can also seek support through a local group that helps victims of sexual assault. Group counseling is one of the best ways to work through a sexual assault. Remember most importantly, it is not your fault.