I think that this shouldn't be a one-time talk, but an ongoing discussion. This is a great opportunity not only about explaining what's going to happen to her body (puberty), but to also incorporate some of your family or religous values (Sex is for two people in a marriage that love one another; We need to respect ourselves and others, etc.)
I would start with the book, "What's happening to Me?" which outlines the physical changes that boys and girls go through during their formative years. We bought that book for our oldest girl. She never has a problem asking me about these things, because I'm open to discussing them with her.
For you, I'd recommend reading the book, "Ten Talks Parents Must Have with Their Children About Sex and Character" by Pepper Schwartz, MD and Dominic Cappello
Very helpful.
Good luck!