nervous about my therapist

If she's a proper therapist she cannot tell your parents what you tell her. Feel free to verify beforehand though, ask her if she plans on reporting to your parents.
Honestly, I'd take this opportunity to get anything you need to off your chest. It can be about why you're taking drugs, or about your parents, or something your friend said the other day, or things you wonder about yourself. Sometimes it's nice to just get a fresh perspective on your situation. And again, if she's a proper therapist she shouldn't be making any judgments about you.
I guess what I'm getting at here is, you don't seem to have a choice about going but you have every choice about what you want to talk to her about. Give it a try, and who knows? It might be a good thing.

Typically with therapy, your therapists can not tell your parents anything unless you are going to seriously hurt yourself or someone else. If it's family or group therapy, obviously there are different boundaries. Don't feel nervous being open with them about your history because that way they can help you more. You might feel a little bit uneasy your first few times, but you will start to feel comfortable the more you get to know your therapist. Sometimes, the first therapist you meet is not a good fit for whatever reason. Some people go through quite a few before they find one who works well for them. It's nice to talk to someone who isn't a parent or friend because that way you are disconnected from them and you don't feel the need to protect them from what's going on. I just recommend being open and going into it as a positive thing even though it wasn't your choice to go. Don't worry, they won't ever yell at you or anything like that. They are very understanding and look for deeper things going on as opposed to just the action of doing drugs etc. Talk to her about what ever is on your mind or whatever you are worried about. She will also ask you questions so you don't have to just randomly try to talk. Good luck and don't stress!