Applying for graduate school is a daunting, very time consuming process. As soon as you decide that graduate school is something you are interested in, you should:
The Psychology office has some books on graduate schools and the application process. The best resource for looking at individual graduate schools is, of course, the web.
Anyone who is interested in graduate school should get to know their professors very well so that they can get good letters of recommendation and complete Independent Studies with one or more professors and/or do an Externship. You also need to find out if the graduate schools you are interested in require the GRE