Your first step should be to not get uncomfortable, dismissive or judgmental your daughter comes to you with questions about sex. Studies show that most teens want to get information about sexuality from their parents and she's giving you a great opportunity to help guide her as her own sexuality develops. So when she comes to you, listen - - and make sure she knows that it's alright that she's curious and that it's fine that she's asking you for information.
As you do listen to her questions, also listen for any hidden issues, concerns or feelings that she might have trouble expressing directly. However, answer the question that was asked. If she asks a pointed, specific question - - answer it. You shouldn't now take this opportunity to sit her down for an hour-long discussion on all areas of sexuality. Also make sure you aren't avoiding her question by not really giving her the information she's asked for. When you answer, use simple, direct language that she will understand. However, once you have answered her question, have her give her version of your answer back to you so you can be certain she's understood. After that's done, definitely ask her right then if she has any other questions she'd like to ask.
You also don't need to wait for your daughter to come to you. In fact, if you don't make the first step to open the lines of communication about sex, she might never know she can talk to you about it. The best time to raise the issue of talking about sex is before she's likely to be sexually active. Look for a natural opportunity to raise the issue, perhaps in response to an issue on a television show. Again, don't make your first talk a comprehensive study on sex - - just an opening so she knows she can come to you. You might not even talk about a specific issue in the first talk. You just want to make sure your daughter knows she can turn to you.