Ahh I'm getting into the exact sime kind of volunteer work! I've yet to bring home the dog I'm looking for, considering my current dogs would not be suitable for this. There is no specific breed, but temperament is obviously important.
Go to the Therapy Dogs International website, it's a fantastic website that describes exactly what your dog needs to know/be able to do to be a therapy dog. Some of the things are: basic obedience, very well-socialized, at least 1 year old, etc. Also, you can become a member of TDI and pass the suitability tests. Then you can contact your local hospital/nursing home/centre and see if they have a therapy dog program there that you can be a part of. Sadly, there are no major therapy dog groups near me, but you could see if there's one near you.
Hope that helped =)
Mista <3