The first thing you should do when you arrive in your first park is to go to Guest Relations and ask for a Guest Assistance Card (GAC). Bring your doctor's note to the Guest Relations window at any Disney theme park and ask for the Guest Assistance Card. The Guest Assistance Card is a special pass designed to allow you to wait in a separate, un-crowded holding area apart from the regular queues at most attractions. One Card is good for all four parks, so you do not need to obtain separate Cards at each park.
It's also important to know that the GAC is not a "front of the line pass." It only allows guests with special needs and their parties to use a different entrance or queue when available, and you will still have to wait your turn, just in a different location.
Speaking of queues, there are many different types of queues at the rides and attractions. Some have a special entrance for special needs guests, and some allow guests to stay in their wheelchair or special stroller through the regular queue. Others require each guest in the special needs guest's party to obtain a Fastpass and to use the Fastpass queue during their valid Fastpass time window. If you are in doubt, ask the Cast Member stationed at the entrance to the ride or show.
TIP: Many guests wear the GAC on a lanyard to keep it readily available to show to Cast Members.