Should I be an elementary, middle, or high school teacher? Qualifications for each?

I know that there's programs only for elementary school teaching ...but what if I wanted to teach middle or high school? I like the idea of teaching elementary school because I'd love working with little kids and it doesn't seem like you'd have a ton of grading to to. I think it would be cool to be a history teacher or an english teach too...or even geography. Can teachers changce what subjects they teach? And what major or education do you need to be a high school teacher? I'm not sure I'd want to teach the same grades or same sunject forever you know?