Two words - education and support.
By educating a girl about her body in all aspects, not just sex, you take away the mystery and stigma. You teach her that sex is not this big bad mysterious thing she should never have until she's married. Teach her about safe sex practices for herself and for any sexual partners she might have. Teach her about STD's, how they're transmitted and what to do if she catches one, but not in the stupid scare tactics way a lot of people like to do.
Get over the idea of 'promiscuity', too. Because guess what? It's a double standard! If a guy sleeps with a lot of girls he's celebrated, but if a girl sleeps with a lot of guys she's shamed. Stop it. If she has multiple partners as long as everyone is of legal consenting age it is no longer your business, even if you are her parent. Instead encourage her if that is a thing she wants to do, and be there for her. Support her if she wants go to on birth control because she's of legal age and sexually active instead of shaming her, or wants to keep some condoms 'just in case' or whatever else.
Remind her of the legal age of consent and that it's there for a reason, but accept once she passes that point she has the legal right. Threatening her or being a controlling parent just leads to secrets, and perhaps even her having sex anyway but in an unsafe manner for a variety of reasons.
So educate your daughter, and then support her. She is far less likely to end up pregnant before she is ready for it, and in the event she does she is also far more likely to come to you for help instead of trying to deal with it herself in a potentially unsafe way.
