I'm glad you came here to ask your questions and I think I can help you out with some of them.
First, the WDW parks don't rent power wheelchairs, only something called an
ecv or
scooter (not the kind of scooter you are thinking of, more like a golf cart). I'm sorry to say that you won't be able to rent an ecv for a few years. WDW says you have to be at least 18 years old to rent one at the parks.
You would probably be more comfortable using a wheelchair if it you get sore or tired from walking or stading in one place. The wheelchairs they rent at the parks are for adults. That would be OK if you are pretty tall, like close to 5 feet tall. If you are shorter, there is a place that your parents can rent a child size wheelchair for you. Here's how to contact the place:
http://www.all-about-kids.com/main.htm
rent children's equipment (including children's wheelchairs)
Phone: (407) 812-9300
Toll Free: 1-800-728-6506 (outside 407 area code only)
Fax: (407) 812-4648
E-mail:
AAboutKids@aol.com
A Physical Therapist might be able to help you borrow a wheelchair if you work with a therapist at school or someplace else.
Some rides are kind of rough and probably would not be good ones for you. Rides like Space Mountain, Dinosaur, Body Wars and Star Tours. They are marked with a little triangle on the park map to let you know they might be a problem. Even if you can't ride some of the rides, there are still a lot of cool things that you will be able to do.
I hope this helps. Write again if you have more questions.
I started writing this before lisapooh answered, but I was slow typing. I'm glad lisa answered to because you can see we have lots of people who cna help you out.