I think its best to check beforehand, you could either
1) Go through a exchange organization, if you are a student between 18-30 I would suggest AIESEC (as I work for this organization The benefit is in general more support, a network of volunteers, introduction to the country and support with logistics. IT might also be better in case you meet difficulties. Of course it will cost a bit of money.
Find AIESEC in your country aiesec.org
2) Contact organizations, schools, orphanages or however you want to volunteer directly. Its not going to cost you anything most likely, but you have less support in terms of network (especially if something goes wrong, it happens that organizations take advantage of volunteers). You should be able to find lists by googling NGOs in Thailand or something like that.