I started comunicating with my daughter when she was a toddler about age appropriate body issues, etc.
She was in 1st grade when I had my complete hysterectomy and she I explained what was happening to her level.. we've always had a strong comunication bond.
In the 4th grade, at our elementary schools, the nurse comes in and talks to the girls because some moms don't or are too embarrassed to do so.
Open those lines of communication now, talk about her changing body, changing moods, what to expect when she first starts her period and if it happens at school the school nurses are completely prepared for this. She should no there is no shame with having her cycle start, her body begin to mature. This is a critical age and you want to help them have good self esteem.
If you don't feel comfortable talking one on one... then talk while you are driving so you don't see her eye to eye or walk in the park. Asnwer her questions the best you can and if you don't know the answer, find out.
Keep in mind that the more you talk, the more she will come to you and wouldn't you prefer her to learn all about the facts from you vs hearsay from girls/boys at school! Believe me, you'll appreciate the bond when she is 17 because she'll need you even more, even though she'll hate you at times.
I've heard that the Care and Keeping of You by American Girl is a good book. I sent off for a booklet from Playtex and saved that for my daughter. We read it together.
Also, she'll probably ask to use tampons and it is ok... they make them so girls can have a life while having a cycle.
Good luck to you... they grow up way too fast!!