What should I major in?

It's almost time for college, and I still have no idea what I should major in. I want to make a difference in the world. I'm a vegetarian and strong animal activist and environmentalist. When I graduate from college I am going to join the Peace Corps to go over to Africa. I would like to major in something that would help me out there. I've thought about majoring in something medical, but I faint with shots so I don't know how that would work out. I'm really good with kids, so I've thought about becoming a teacher, but I don't know if that could take me as far as I want to go. I just want to be able to be the change I wish to see in the world. Any advice?