Oh wow, you'd better do it very soon, af is on it's way for sure!
Where to start - hmm, well I would get some good books - for you to read and then for her to keep and read and ask questions. Give her the basics in simple terminology, but be truthful, make sure you let her know how special these changes are.
I'd get her a cute zip up type small bath bag to put pads for school in, something discreet to stay in her bag just in case - also talk to her about what to do if it happens at school and who to go to - is her teacher female? Do they have a school nurse?