I think it depends on how you want to help. Neurology, psychology, occupational therapy, and psychiatry all come to mind, whether it's research, diagnosis, or therapy. Other options could include law (disability or discrimination law), as well as the liberal arts (sociology or anthropology). You could even be a social worker or a writer--it all really depends how you develop, adapt, and market the skill sets you learn. I would be hesitant to recommend a liberal arts degree though--it's much harder to get a relevant job without a science degree.
In all honesty, we need people to spread awareness, but it's hard to do that as a career. People usually listen to people who are already highly accomplished in a relevant field.