I agree with the suggestions above. Have you gotten your Canine Good Citizen certificate yet? I believe this is a great preparation step towards Therapy Dog work. Then evaluate the therapy dog evaluation test and work on the areas that you need to, ie practicing around wheel chairs or crowds etc.
If you find a local therapy dog group near you, talk with them and maybe even see if you could shadow one of their visits (without your dog) to see what it is all about.
My rottie and I have done this work for a year and a half now - extremely rewarding and the bonding experience with your dog is unbelievable - who thought it could get deeper but it does.
The link to the Delta Society Pet Partner Program is below and will tell you the steps to testing and registering with them.
Delta Society - Pet Partners Program