How much training have you done with your dog?
She'll need to know all the basics (sit, down, stay, leave it, loose leash walking in a crowd, polite greetings, etc.) and also be OK with all sorts of people, different locations, as well as medical equipment (wheelchairs, walkers, IV stands, etc.)
Take a look at these sites to get an idea of testing requirements and which group you want to join. There are also other organizations out there, but these are the bigger ones. Therapy Dogs International Pet Partners® Therapy Dogs Inc.
IME, while some places do let people and dogs come in without certification, it's best to test with and join a group like one of the above because insurance is typically part of membership.
If you feel your dog is ready then you can search for upcoming tests on the above sites. If you feel that she needs additional training then find a good Positive Reinforcement based trainer in your area and sign up for some classes. Work towards and get your dog's CGC (or equivilant if not in the US). Therapy dog tests are often quite similar to the CGC so it's good practice and gives a good idea of whether or not a dog is cut out for therapy work.