Verified answer

You can sign up at a school like Bonnie Bergin's Assistance Dogs Institute, which has recently changed it's name to Bergin's University of Canine Studies. http://www.berginu.org/ There are a few other schools out there but hers is by far the most recognized and respected.

Alternately, find a local program to volunteer with and work your way up the ranks. This webpage could help you find a program near you: http://servicedogcentral.org/content/node/262 It's actually intended to help people with disabilities find programs to get a dog from, but in the process it has a pretty good collection of resources on finding programs by state.