Travel for cheap? South America, Africa, China?

I am interested in traveling to somewhere some time between the summer '09 and summer '13 (I graduate HS in 2009, and Uni in 2013)

I'm looking for an experience to embrace the culture, help people, and make a change. However, I can't afford to go on my own. I'm not a religious person, so I'm not sure if a mission trip would be right for me. I just want to experience life outside the US.

With that amount of time to plan, I am positive you can get funding of some sort. Especially if you want to plan to do a study abroad or international internship at the end of your undergraduate. If you are a talented student you can definitely get a scholarship or a spot working on research with a faculty member.

Based on the substance within your question, you seem to be very focused and ambitious and are likely to pull off a great trip.

Good site to get you directed:
http://www.ngoabroad.com/