First, find a group in your area that does it. Either contact local nursing homes or try your local kennel club. They can refer you. You can also find a group by finding an evaluator.
Most groups either hold a class or refer to a class that does some basic obedience and gets the animals use to things like wheelchairs and walkers. Then your dog is evaluated by somebody certified to do so.
You want to be with a group that is insured so I wouldn't just do it on my own.
http://www.deltasociety.org/
http://www.tdi-dog.org/
These are the 2 biggest therapy dog organizations in the U.S.A.