department of psychology
The most successful applicants to graduate programs have strong GPAs, strong Graduate Record Exam (GRE) scores, strong recommendations, and some relevant clinical and research experience. A relative weakness in the above can sometimes be offset by some special strength or background.
GREs are very much like the Scholastic Assessment Tests (SATs) you took in high school as part of the college application process. This sort of standardized test can be very frustrating if you tend to test poorly, but remember that taking these tests is a learnable skill. Many people have found one of the commercially available preparation courses helpful, although these courses are expensive. At least buy or borrow one of the practice books. Plan to take the test only when you are well prepared; don't count on taking it three times just to see if you can improve your scores.
There are national administration dates for these exams and "special" administration dates that occur quite frequently. Consider taking the GRE on a "special" administration date if it will give you more time to prepare. (You may have to pay for this privilege.)
As a hint for the Psychology Subject test, re-read an introductory psychology book just before taking the test. Another hint is to take the test on a different day than the aptitude tests, just so you are not tired.
Some schools require other less well-known tests, such as the Miller Analogies Test.