For dental volunteering specifically, my suggestion would be to look for a not for profit clinic, or public health clinic. There are advantages to one over the other, but both are great volunteer experiences.
In my experience, the advantage at a not for profit clinic, is that many are younger dentists serving their few years to pay back their grant money. They are more willing to slow down to teach you what is going on, and explain the subtleties of procedures, and the nature of community health work. In a city run public health clinic, the patient load is overwhelming, and you'll need to pack your running shoes. It'll give you an idea of the pace of a high volume dental office, and really let you know if it is for you. The dentist here was too busy to explain or talk much, and it was really about taking initiative to help out.
My advice would also be to get looking ASAP, because even though you're volunteering, it takes some time for them to verify your immunization records, references, and to match you up with a dentist that might have time for you.
