There are a number of organizations that rank programs (see Helpful Links), but you might find it more useful to talk with faculty members in your field. Chances are good that they will be able to point you in the direction of several schools that could be a match for you. Additionally, faculty members can point you in the direction of professional societies in your field. These organizations often have lots of resources for undergraduates interested in advanced studies. Look around on online. Try to find departments where several professors are doing work that interests you.
As you begin collecting information on possible programs, you will want to consider:
You should take the GRE at least one year before you plan to begin graduate school (for example, you should take the GRE in the fall of 2011 if you want to begin graduate school in the fall of 2012). That said, taking the GRE sooner gives you more flexibility to retake the exam if you are unsatisfied with your score. Some graduate programs provide GRE deadlines; thus, you should be familiar with the requirements of each of your prospective schools.