Like JudithM mentioned, there are a lot of attractions you can see without leaving a wheelchair or
ecv.
Click here for a link to mobility disability information on the DIS. If you scroll toward the end, you will see a list of attractions where you can stay in a wheelchair or ecv. ones you need to transfer from an ecv into a wheelchair and ones where you have to transfer to a ride car.
The Guidebooks for Guest with Disabilities that JudithM mentioned are available from Guest Services. There is one guidebook for wach park. They don't give a lot of information about how you board each ride, but they will tell you if it is, for example, a slow boat ride or a turbulent thrill ride.
For boats rides you need to step down onto the seat of the boat and then step down again to the floor to sit down. The boat rides I can recall are:
Small World, the Mexico ride - wait for the ramp car: you can stay in a wheelchair or use the ramp car to walk on the boat
Pirates, Maestrom in Norway, Jungle Cruise, Splash Mountain - you need to step down and there is no way around it.
You can ask the first CM you see at a ride how boarding is done. They will know whether the moving walkway can be slowed or stopped to help you get on. Sometimes getting on at the exit allows a little extra time to get on. If you ar eusing a wheelchair or ecv, you will be automatically directed to the exit for rides where that is appropriate.
While you are waiting at the boarding area, watch the boarding process. That will help you decide how to get on and off yourself. Most of the rides at WDW board in similar ways, so once you have been on a few rides, you will recognize things that will help you.
Are there specific rides that you are interested in? Just list them here and I'm sure you will get some responses that will help you.