If you're trying to apply for disability in the United States, here's some helpful hints and resources that I've found invaluable. The downloadable files contain the RFC forms.
If possible, get a written diagnosis from a doctor that details what your disability is, when it started, and how it impacts your day to day life.
Residual Functional Capacity Forms help detail how your disability impacts you in specific ways. A doctor needs to fill this out for you. There is a form for physical disabilities and mental disabilities.
Personal letters from employers, teachers, parents, coworkers, friends and family can help to explain how your disability impacts your ability to support yourself.
Be sure to include records from hospitalizations, voluntary or involuntary.
If your disability has impacted you in legal matters, be sure to include that information, as well as police records if that pertains to your case.
A lot goes into preparing a case for disability and it will be helpful to prepare a binder that contains all the pertinent information in a way that's easily accessible.