I'd start here:
http://www.usnews.com/usnews/edu/beyond/bcrank.htm
Request some viewbooks and applications first (i.e. get on some mailing lists). Once you have an idea of what exactly you should be considering in a school (program compatibility, reputation of program, acceptance rate, average test scores, fellowship availability, etc.), it will be much easier for you to decide where to apply. My best advice is that you visit the campus AND talk to both faculty members and current students about the program, and even alums if you can. While everyone's experience is very unique, it helps to get as much insight from a diverse group of people as possible.
Good luck!