If you don't get above a certain score, they may not even look at your application. Graduate Study in Psychology lists the average GRE scores for students who are accepted into a program. A few less competitive graduate schools may not have a cut off score or may not require you to take the GREs at all. It is very unwise to take the GREs cold. Prepare for it.
Bookstores sell manuals that describe strategies for taking the test and provide sample exams.
There also are classes you can take, such as the Stanley Kaplan preparatory courses . A good way to study for the Advanced test in psychology is to get a good intro psychology textbook and memorize as much of it as you can. There are books that can help you prepare for these exams. The Dean's office also has applications. For more information, here's a link to a PsychWeb file on the GREs.