The Canine Good Citizen program is separate from Therapy Dog certification though many Therapy organizations base their requirements on the CGC.
The CGC requirements are detailed on the AKC's website,
http://www.akc.org/events/cgc/program.cfm. You can easily train all the requirements for this at home but the test has to be issued by a certified evaluator. The advantage with taking a class is that you will work on the specific skills with others in the class. It provides a controlled environment for the tests where dogs have to be within close proximity to one another, such as greeting another owner.
For therapy dogs, Therapy Dogs International (TDI) requires dogs to be over 1 year old before they can take the Therapy Dog certification test. (Check with whatever organization is certifying your dog for their specific requirements.) Many CGC evaluators are also certified to issue the Therapy Dog test. In the case of TDI, their requirements are the same as a CGC
plus a few extras such as noise, "leave it", and reacting to someone who has fallen.
The best thing to do for therapy dog certification is to contact the organization you are interested in and inquire. Some hospitals, such as my local Children's Hospital, require TDI certification plus their own certification.
I hope that helps.
