I'm 17 and i really want to help out with kids in underdeveloped countries, africa or south america. How?

One of my friends went to Zimbabwe to help with kids over the summer. The story, and just seeing photos of her with the kids made me really want to experience something like that. I'm just curious if anyone has agencies they've gone through to do something like that. Possible cost? What I'd have to do? I'd really appreciate some good info because I really want to do this, the right way. Thank you! You're answers are greatly appreciated!