I'm glad you're interested in doing therapy work with your rottie, and I'm glad to help you get started. First of all, check out these two websites:
www.deltasociety.org for Delta Society (see Pet Partners)
www.tdi-dog.org for Therapy Dogs International
The Delta Society requires you to complete a short home study course as a requirement for certification. You and your dog must also pass a skills and temperment evaluation. The web site will also provide you with the names of certified evaluators in your area. Usually the evaluators will answer your questions and may even mentor you through the process.
TDI also requires a skills and temperment evaluation for certification, but I don't know what their other requirements might be.
Therapy work is hard work for both human and dog - but it's also very rewarding. Please feel free to ask any questions you may have and/or PM me if you like !