Certain carriers deal with disabled travelers better than others. Some of the best airlines for this are: Air Canada, Air France, British Airways, Continental Airlines, Delta, EasyJet, Quantas, Southwest, United Airlines, and Virgin Atlantic.
Airlines required advance notice to accommodate and offer additional services for your disability. In addition, you should let the airline know as soon as possible and get their confirmation that they will be able to assist you. If they cannot, it's best to find a different airline.
Most airlines have specific seating for disabled travelers, however it is still important to request a seat that is suitable for you and your needs.
Each person's disability is unique and it's important to consider how long of a flight you are able to handle. It may make more sense to book flights that are not non-stop, especially if you cannot use the toilet on the plane.
If you need help to enplane, deplane, or anything else, be as specific as possible regarding how you would like/need the help. It's also okay to decline help if you are able to do it all by yourself.
If you need help getting to the boarding gate, feel free to ask for an unticketed individual to assist you. Most airports have "Airport Escorts" that would be happy to help you.