I have a 12-year-old son with ADHD who also was a fairly late diagnosis. Congratulations on being pro-active about this...you're already way ahead of most parents in the early stages of this. Here is the advice that I can offer you from a "been there, done that" perspective.
1. Find out if your school district has a family counseling/resource center. I live in Jordan School District, now becoming Canyons and both districts offer this service. It's a great resource...they offer "intake assesments," short-term and long-term counseling, parent classes for coping with ADHD and a fairly good library of materials on the disorder. All of this is completely free if you are a member of the district. The classes offered by Jordan District are free even if your child is not part of the district; they are offered twice a year and are extremely helpful.
2. Knowledge is power. Go to your library or bookstore and find a book about ADHD...whatever book looks like it works for you. The more you understand the disorder, the better advocate you will be for your child. Read, read, read!
3. You can also join C.H.A.D.D. -- a parent support group-- for a fee and receive their magazine on a regular basis. I find this publication timely, informative and helpful on those days when I feel like nobody around me really "gets it."
4. Once you're "armed" with information, arrange to meet with your school counselor and your child's teachers. Communication is absolutely critical! The best way to ensure your child's success is to work for her good as a team.
5. Most important...you're going to have to develop a thick skin. There is so much stigma associated with this disorder...people who know nothing about it think they know everything. If I were to tell people my child has Diabetes they would treat me with compassion; when I say my child has ADHD they come at me with judgment. Just get used to people saying insensitive things--understand that they are just not well-informed and learn not to take it personally.
6. Hang in there! The fact that you're asking this shows that you're a good mom.