Well, I haven't had to deal with this yet, but I'm the oldest of 4 girls do I'll just tell you how my mother handled it with us.
Be matter-of-fact. Tell her calmly and clearly what to expect. Whatever you do, don't be embarrassed. You don't want her associating puberty with something that should be embarrassing or shameful. Be clear that it is something normal that all girls go through and it's nothing to be ashamed of or embarrassed about. Tell her what she can expect that is normal and not normal and to come to talk to you if she's worried about something. Give a lot of details and explanations.
Chances are, she already knows a whole lot more than you'd think. You'd be amazed at what kids learn from other kids at school. Some schools also give a non-sexual, human physiology lessen in class in elementary school and it covers stuff like menstruating and puberty. If she's already had this type of class, then she already has some idea of what to expect. If she's been shopping with you and has seen you buy female products, then she more than likely knows what they're for even if she's never asked you about them.
Take her shopping for training bras. Just you and her. She'll be thrilled to not only be shopping, but also that it gives her private time with you. You can even have "the talk" while driving to the mall.
You might want to start keeping a bag of pads designed specifically for young girls in the bathroom. For some reason, girls are starting their periods younger and younger. If she's already starting to grow breasts, chances are the periods won't be far behind. Let her know they're there if she ever needs them. Pharmacies also offer midol for teens now. If she gets cramps, get her some.
Good luck! The teen years are a mixed blessing, according to my mother. They're fantastically wonderful because daughters seem to become more attached to their mothers, whom they now have way more in common with (especially if they were previously daddy's girls) and they can also be horrifically awful because now they're hormonal and have mood swings and want to be even more independent.