The best thing is to talk to her about the changes in her body. Advice her on how to carry herself now that she is hitting puberty. Explain to her about the menstruation cycle and how to take care of herself when on her period.
Sit down with her and talk to her about what's going to happen and that it's all a good thing. There are some books that you can get her like "What's Happening to Me?" by Peter Mayle that explain and show the changes that she's going to go through.
I wouldn't full out give her the baby talk, bits and pieces. Till you think she is at a sutiable age for the "talk". I was 11, and started my period at 12. It all depends on what you want to do. I would explain the changes that are going on ect. But, remember she will have a class in 7th grade about the systems.
She shouldn't get her period until at least 12. I got mine after my 12th birthday. Talk to her about the changes in her body. She might feel uncomfortable, and that's perfectly normal. Have a female doctor talk to her about it if it makes her feel less uncomfortable.
You can call her school and see if they show a movie or anything about puberty. my school did for 5th graders. You can also do what my mom did: she talked to me about what happens to a girl's body during puberty, showed me how to use a pad and gave me a game plan for if I got my period in school (tell my teacher in private and go to the nurse and tell her) As someone who got her first period when she was 10, I can tell you that just knowing she can talk to you about this stuff and not to panic when she does get her period is really all she needs. :)