I too have a Service Dog, a retired lab and a little guy in training currently.
Cannot stress the socialization enough. People are unpredictable and you need to know that your dog will be unflappable in the face of anything, same for a Service Dog. I have been out in public with my retried SD and dropped several packages of maxi pads on his head, ran over his foot with the cart, had little kids hurl themselves at him and stick their fingers in his nose, pull his ears, etc.
They need to be fine with EVERYTHING. Loud sidden noises. Being touched anywhere by anyone at anytime.
Other animals. ALL types of people, white, black, asian, everything in between. Babies, and the very elderly. Mentally challeneged people with slured speach who may be prone to outbursts, Parkinson suffers who may have very shaky unsteady movements while trying to pet the dog, etc.
Walkers, wheelchairs, scooters, crutches, canes, you name it.
They say that only 1 in every 500 dogs is suitable for Service Dog work, but I suspect the same number apply for Therapy Dogs, maybe a little more like 2 in 500.
You should have the dog that you are interested in using temperment tested to be certain that he/she is stable. Then I would proceed with normal obedience training and obtaining a CGC from the AKC. Then move on to thinking about the TDI.