Thinking of volunteering abroad and need some help!!?

Hey, so my friend and I are thinking of volunteering in some developing countries next summer and i was just wondering if anyone has had any experience with this??..... We are both 16 and I know there is a few different companies that have special programes for 16-19 year olds and that kind of thing. The places we were thinking of going are (only 1 but we cant decide where to go) Fiji, Cambodia, China, India, Nepal, Sri Lanka or Thailand)..... Any tips, advice or general comments would be good!!....