My pug is a certified therapy dog with Therapy Dogs International. There are other therapy dog organizations you can test with and join. Delta is the other group that comes to mind.
This is TDI's site
Therapy Dogs
This is Delta's site
Delta Society - Improving lives through Service & Therapy animals
I started out taking Roxy to obedience classes and after intermediate obedience, she took and passed the CGC. A few months later a TDI evaluator was testing dogs at a local pet expo. Roxy and I tested and passed the evaluation.
Passing the CGC is a great start because all parts of the CGC will also be parts of a therapy dog test.
AKC's Canine Good Citizen (CGC) Program
A dog testing to become a therapy dog will also have to prove it does well around wheelchairs, walkers etc.
This site is a brochure containing the different aspects of the TDI test.
http://www.tdi-dog.org/images/TestingBrochure.pdf
I would suggest that you and your dog test with and join an orgnization because they provide liability and accident insurance for all volunteers and dogs. If you aren't a certified team with an organization, many places will still allow visits but you would be financially responsable if something happened.
Something else you should know is that your dog must be
at least 1 year old (thats a rule with TDI- I'm not sure about other groups). You will also need to pay a yearly registration fee as well as provide records (also every year) proving that your dog:
1)is up to date on all vaccinations (DHLPP & rabies)
2)has had a negative fecal exam within the last year
3)has had a negative heartworm test within the last year (or proof the dog is taking preventatives)
4)has had a health exam within the last year.