I want to travel to South America to teach/help/work with children?

This has been a dream of mine for many years. I am only 21 years old, but I feel the overwhelming need to travel to third world countries and visit and help and care for the young children that live there. Whether it's teaching English, or working in an orphanage.. I want to go and help in any way that I can. I have been researching for a few years now with little luck or knowledge on how to go about this. I have come across many different volunteer/internship opportunities to travel to my main choices (Brazil and/or Peru), however I am not sure which are legit.. and I have a feeling there MUST be cheaper ones out there. All the ones I am finding do not include airfare, and are a good few thousand MINIMUM to stay for a week or two. And that is not including food either. The reason it has taken me so long and I actually haven't made a move in doing this trip I want to go on.. is because my lack of money. Lack of money means I can't go, so why bother? But maybe if someone could help me to find a decent company or group to go with that I could trust and feel confident about, with a decent price, maybe then I would be more motivated to actually make this trip happen. Does anyone have ANY information that might help me? This is a true dream of mine, and any help with different programs, or ways to raise money for it.. anything at all will help me please.