Whatever you do, DO NOT ground her and start the whole alienation/punishment cycle that once is started is so impossible to break.Nor should you stop her seeing her friends male or female, that would just be parental suicide.
Keep it light and easy and keep all routes of communication open at ALL COSTS. She may or may not have experimented with sex yet, and if she hasn't then it's a great opportunity to broach the subject lightly and easily and if she already has she may be more clued up than you supposed. Just bring up the subject of teen sex and let her know that there is nothing she can't tell you whatever it is and then gently outline contraception and get chatting about it, find out what SHE already knows and above all maker her HIV/AIDS aware if she isn't already. From her responses you'll end up knowing if any sexual activity has taken place and you can take it from there, but PLEASE don't just assume she's been doing what you're worried about as she probably hasn't and will be mortified if you confront her and she's not done anything.