Thanks for your question. I imagine there are many, many parents with children your daughter's same age who have been struggling with the same issue, and I hope that they, too, can benefit from the response you receive to your question.
Your own comfort level is important here, but I believe that your daughter is old enough that you can begin to set a foundation for her that is based on accurate information. Your daughter is at an age in which her body is about to change (or has already begun changing), and she would benefit from your teaching and experience on this subject. Although many schools provide some basic information about puberty, children still have many questions, and if she doesn't hear this information from you, she will go to her peers.
I would begin by looking for an opportunity to initiate a discussion with her. If she has already expressed an interest in topics related to puberty or sex, I would find a time to talk with her when she is relaxed (at dinner, when she is getting ready for bed, on the way home from school) and raise the subject again. You can say something like, "I was thinking about a question you asked the other day, and I have some more ideas I wanted to share with you..."
If she hasn't raised the topic with you, you can look for opportunities that are presented in your everyday life. When you see a commercial about feminine products or a TV show that mentions sex, you can turn to her during a commercial and broach the subject with her. Start by asking her what she knows. You can say, "Wow, I was watching that commercial, and I realized that we have never talked about tampons. Do you know what they are?"
In my experience, I have found that preteens enjoy hearing their parent's stories and experiences with puberty. If you feel comfortable, let her know what age you got your period and how you felt about it.
You might also want to discuss with her the different ways in which people in society will react to her growing adult figure. Help prepare her for the whistle calls of men and the pressure on women to look a certain way.
It is also appropriate to begin talking with her about sex. Remember, you want her to feel like she can talk to you about difficult topics like this. Keep it simple and feel free to share your values along with the facts.