This is a scary subject and how to handle it can mean so much to your daughter. I remember watching a show on Oprah a few years back about two 14 yr olds saying "they are ready to have sex". I believe they had Dr. Laura Berman on the show. She didn't tell them not to but asked them questions and made them answer to make up their own minds. She would ask them questions like:
Why do you think you are ready?
What would you do if you got pregnant?
Are you ready to be a parent?
How would you feel if you two were to break up (w/or w/out pregnancy)?
These questions (and more I am sure) made them think. While the boy (of course) still wanted to have sex the girl decided not to...then wouldn't you know it they broke up shortly after. I tried finding the interview on the web but couldn't find it. I did find is the show "how to have the sex talk with your kids".
http://www.oprah.com/relationships/Talking-to-Your-Kids-A...
http://www.oprah.com/relationships/Download-Dr-Laura-Berm...
This is a link for her hand book on how to talk to your kids and other stuff.
I have three girls myself (6, 8 and 12) and we use the book series "Gods design for Sex”. There are four books and they are all age appropriate. You can even read the books with your children, they read what the child role and you read the parent role. It's like they are asking your questions w/out being embarrassed or not knowing how to ask. And us parents saying enough...but not too much for their age.
Since we have opened these books, my girls have been open to me. My oldest hasn’t started her menstrual cycle yet but tells me every detail of what's going on with her body in that way. This is TMI but the other day when I was in the kitchen preparing food she came up to me to tell me that she had a lot of wetness and discharge all day long. I told her jokingly that there is a time and place to talk about these things and her comeback was “but mom, you said I can talk to you about anything anytime”. I just had to laugh.
http://www.christianbook.com/gods-design-for-sex-books-re...
I hope this helps.