There are three big national organizations:
Delta Society
Therapy Dogs International
Therapy Dogs Incorporated
Rosco and I are registered with Delta Society (certification is not applicable because Delta doesn't train the dogs only registers them based on passing an evaluation).
I believe most if not all of the above offer liability insurance as part of membership--which is nice in case anything bad ever happens (unlikely but ya never konw). And most, if not all, offer training classes or workshops.
At least with Delta (maybe all?) their training classes are NOT required. However, in general the pass rate is lower for those who have not been through classes. And yet both Rosco and I and three other local teams passed in the last year without having taken classes...so it is TOTALLY doable if you train your labradoodle or goldendoodle well and he/she is basically polite and non-aggressive =) The main thing here was that classes were 3 hours away...and while we still had to drive that distance to take the test..no way was I driving that far weekly!
I'm attaching the Delta two-part evaluation for your perusal below. None of it is terribly hard. For us the hard part that we had to train for and practice LOTS for the weeks leading up to our test was the greeting another person with a dog. And the "leave it" which I thought we'd do fine with ... was our sticking point on the test. I was prepared for them to bring out a stuffed animal or large car or something.
They brought out a 6 inch plastic horse. Rosco lost his self-control entirely...I'm POSITIVE he thought it was a real animal of some sort (skinny squirrel?). We managed, but just barely. Thankfully we're not faced with 6 inch horses "on the job" too often--LOL