How to find a Grad School
You probably will want to consider the reputation of the field of study at the school not just the department as a whole. Some schools are not strong in every field in the department but are superstars in particular fields. For example, Brown University is not considered a big computer science school, but it has a great graphics program. For PhD programs, you will probably want to look more closely at professors in the field of study. How many professors are there in the field of study? Can you find professors with whom you'd like to work? This is important because you will need to find a thesis advisor at the university and the more choices, the better. If you cannot find a single professor who you would like to work with, then scratch the school off your list. There's no point in applying to a school if you cannot find an advisor.
Some schools and departments provide funding for all graduate students and some don't provide any. If this matters to you, find out. If the school does not guarantee funding, then you might want to look for external fellowships.
Some other things you might want to consider are location, qualification exams, student life, etc.
Your best resources are other students in the major and faculty members in the field you wish to study. Faculty members can tell you what the top schools are. Professors who know you well can help you create a list of schools to apply to. Professors will have a better idea of your reach and safety schools.
I've been told to look at the most popular schools of authors of research papers in the field I wish to study as an indication of where the most research is being done in this area. I have not actually done this, but it may help you.
http://www.phds.org/rankings/: a free website that ranks schools based on your priorities. Data is from the national research council.