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.
Definitely look into the above resources. In north Jersey there is a place called St. Huberts. They do testing and certifications. Then you're free to apply to different groups that use therapy dogs. St. Barnabas Health Care System has a group called Paws for Patients, it's connected to the hospitals and hospice services in their network.
If you live in a small town or rural area it can be as simple as contacting a hospital or nursing home and offering to volunteer.
I wanted to get involved with Sophie several years ago but realized she wasn't going to pass the obedience tests and isn't really too enthusiastic about strangers touching her. One part of the test that would have been impossible for her-the dog has to sit/stay while the handler walks to the other end of the room. Another command important for passing the test is "leave it".
The dog has to be willing to walk past a food item without approaching it.
If your dog knows basic commands already you'd probably do fine though.