It is a shock to find out that you might be different from other people, so it's not surprising she had a reaction to hearing the idea. Obviously you need a diagnosis before you can get into details about it with her, though. If she does have ADD, there are books for kids and teenagers about it.
Also, tell her that Michael Phelps has ADHD! I read somewhere once that ADD is more like "differently focused" than "lack of focus" because my son can definitely show hyper-attention when he is engaged, and hyper-attention can be a very good thing when it's directed - like to winning gold medals!
As far as the evaluation process is concerned, you can try talking to her about how every person learns differently, and the evaluation is to help figure out what will be the best way for her to learn.
You have the option to request an evaluation through your school. I don't know much about how that works, but someone at the school district office should be able to direct you, or find out who at your school leads the Resource Team, and that person should be able to help.
If your insurance will cover it, we had our son evaluated by a behavioral neuropsychologist. It was about a half-day of tests and without insurance it would have cost over $1000, but we had several different areas of concern and the psychologist and counselor who had seen my son weren't exactly sure what the diagnosis was, so we felt we needed the full workup. I believe if it's a classic case with straightforward symptoms, a pediatrician can diagnose ADHD.