Hi Kimi,
If you go to the main forum and scroll further down, you will see that there's a whole sub-forum that covers all aspects of Therapy Dogs and Service Dogs. That would be a super place to start and post questions specifically about Therapy Dog work.
A really good place to start would be by working on basic obedience and taking the AKC Canine Good Citizen test. A lot of Therapy Dog tests are based on or similar to the CGC, so taking that would give you a pretty good idea on whether there are any specific things you will need to work on in regards to Therapy Dog testing.
Then the next place to go is taking a Therapy Dog test. There are national organizations, such as Therapy Dogs International (TDI), Therapy Dogs Inc. and the Delta Society, but there are also smaller, local groups. Which you want to go with really depends on you and what your plans for Therapy Dog work are.
A local group can be a better choice if they have an established program at a local hospital or old folks' home so you wouldn't find yourself trying to get started on your own, you'd be able to get started with group visits. Since Therapy Dog work is as much about the handler (it's the handler who introduces the dog, starts conversation, stops interaction when it becomes too rough, etc.) having experienced dog/handler teams with you to help you get started can be a great help.
Most Therapy Dog organizations require that your dog is at least 1 year of age before you can certify and get started. (I would love to see many of them make that 2 years, so the dog has some time to mature, personally.)