Narrow your interests. Psychology is a broad field and graduate programs can take many different forms. Think about what, exactly you would like to study.
Is there a specific theoretical orientation you are interested in? Theoretical orientations include family-systems, cognitive-behavioral or humanistic points of view. This is how the overall program will view human nature. If you don't agree with a particular perspective, you will likely be very unhappy in a program that subscribes to it. When you begin looking at rankings, you will be able to sort out programs via theoretical perspective.
Discern how long you would like to study and what you would like to do for work.
Programs are ranked by categories including graduate and Ph.D. tracks. Ph.D. level psychology programs tend to be more rigorous, involve teaching, and lead to careers in research. Master's degree programs tend to have more application and train students in counseling or therapy skills. Rankings are also sorted into types of programs, for instance: social work, counseling psychology, marriage and family therapy, educational psychology and gerontology. Knowing your area of interest will help you pick subcategories and use ranking systems to your advantage.
Consider what criteria have been used to rank schools. Different organizations use different criteria to rank schools, and these should match your own criteria for deciding on a program. For instance, some rank schools based on popularity alone, with rankings based on reports only from within the academic community. These criteria include how many grants a school offers, number of foreign students, graduation rates and the research capabilities of specific departments. Some allow you to choose criteria and create rankings based on your interests such as program quality, publications by faculty, quality of research, school safety and social atmosphere.