There are two ways to volunteer in Thailand, paying for it or not paying for it. A new form of tourism has developed in Thailand where volunteers pay for a place in a program. The organizers explain the need for payment to be able to fund the different projects, pay locals to show the volunteers around, housing for the volunteers among other costs. The paid programs usually last for two weeks while free volunteer programs last longer thus having a higher value for the communities.
If you are interested in volunteering there are lots of different organizations offering programs online.