There are several National organizations registering dogs for therapy work.  Our members belong primarily to either Alliance of Therapy Dogs (formerly known as Therapy Dogs Inc.)  https://www.therapydogs.com/  or  Bright and Beautiful Therapy Dogs  http://www.golden-dogs.org/​   These national volunteer organizations are dedicated to regulating, testing, insuring and registering therapy dogs and their volunteer handlers for the purpose of visiting nursing homes, hospitals and wherever else therapy dogs are needed. 

We have attached a link to the National Organization, Alliance of Therapy Dogs which gives more information about the process of getting your dog certified. On the link is a very informative video that shows various dogs being tested for certification, and goes step-by-step through the testing process.   The test for most national organizations would be similar.
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https://www.therapydogs.com/join-therapy-dogs/

All types of dogs are used as therapy dogs.  Each national registering organization has its own criteria for training, evaluation and testing.  

 In order to be considered as a Therapy Dog, a dog must be at least one year old and have a good disposition. Both dog and handler must be willing to go through training, testing and evaluation by a regulating organization. A health record or equivalent form from one of the certifying organizations must be completed and signed by a licensed veterinarian. Dogs must be clean and well groomed for their visits to facilities.