R.,
I have raised three daughters, now 20, 18 and 16. I started out by going to one of the local bookstores and finding a book that answered a lot of questions that your daughter has already asked or will ask in the future. Find one that falls in line with the truth and practice how you will tell her. I know it sounds silly, but the more comfortable you are with the explanation, the better it will come off to her.
One piece of advice, develop a really good poker face so that when she asks you questions, you can answer her honestly and without looking uncomfortable. This will open up the doors to a lot of communication down the road. If she sees that you are comfortable talking about things, she will be more likely to come to you when she wants to talk. Also, keep in mind NOT to give her more information that she is really asking for. When I felt that the conversations were getting close to an end with my girls, I would ask them "Did this answer all of your questions or do you want to know more?" Sometimes, my answers were enough (for the time being) and other times, they wanted more information. Just try to make sure that she knows that she can come to you with ANY questions or problems without judgement. By just having that pathway open, you will have a much more communicative teenager. Good luck!
C.