Both of my Rottweilers were therapy dogs and I used to be an assistant evaluator for St. John Ambulance Therapy Dogs. While basic obedience is definitely needed, the certification evaluation tests more for correct temperament than anything.
I would work on socializing the heck out of your dog, to all ages, sexes, shapes, colors and sizes of people. She needs to be comfortable around all types of equipment you'd find in a hospital or nursing home setting, like wheel chairs, canes, walkers, etc. She should be able to take a bit of rough handling, anywhere on her body, without reacting negatively.....not everyone you visit will have the physical or mental capacity to pat her gently. She will need to be able to withstand loud noises and large groups of people all trying to pat her at once.
As you can see, some of the above you can train for but some of it depends on your dog's temperament. I haven't been with the organization for a few years now, but when I was, if your dog didn't make it through the evaluation the first time, you are were able to go back and retest a year later. The only exception was if your dog failed because of aggression towards a person or another dog, you were not allowed to retest.
You can't test before a year of age because they want to make sure the dog's temperament isn't going to change. With puppies going through fear stages, etc, it makes it hard to judge but by a year, a dog's basic temperament is usually apparent.
Hopefully this info is of some help.