EcoTrotters has got an article on volunteer tourism. You should take a look at it. It tells you what you should consider, depending on whether your main goal is volunteering or vacationing, with a littke volunteering on the side.
There are so many places you can volunteer in, that the first thing I suggest you do is choose a destination, and then research the possibilities available. Also, try checking out voluntourism.org. This site gives you tons of tips, and helps you organize yourself, and determine whether or not you´re cut out to be a volunteer traveler. GOOD LUCK!!