Once you're certain that grad school fits into your career and life plans, you need to find out as much as possible about the program you have in mind. Early in your junior year, begin to explore schools offering the type of program you want. There is no single reliable ranking of graduate schools.
National rankings do exist, however each is based on different criteria.
Therefore, it may be more meaningful to talk to faculty in your field and see which professors are doing research and publishing. While actual rankings may be somewhat misleading, comparative information about various programs is readily available. As you attempt to gain an overview of the many graduate and professional school programs available, you may find the following directories particularly helpful. They are probably available in your Career Resources Library. Many profiles list faculty and their research interests.