Graduate Schools look at Grade Point Average (GPA), Graduate Record Examination (GRE) scores, recommendations from faculty, and relevant experience (research and/or internship).
GPA. For a Master's program, a GPA of 2.5 to 3.0 is likely to be adequate. The nationally competitive programs can select students with the highest grades, so a GPA of 3.0 or higher may be needed. For a Doctoral program, a GPA of 3.5 or higher is recommended. Therefore, it is important to make good grades through out your undergraduate career. Some Graduate Schools look at the junior and senior years more closely than at the first two years, and most will look at the difficulty level of the courses you took. For example, a "B+" in Chemistry may be more valued than an "A" in Flower Arranging 101.
GRE. Many Master's programs do not require the GRE, but the nationally competitive ones do. All Doctoral programs require the Aptitude (Verbal, Quantitative, Analytical) part of the GRE, and some may also want the Advanced Psychology GRE as well. It is recommended that you take the Aptitude part in the spring or summer between the junior and senior year, so that it can be re-taken if necessary.
The Advance Psychology part should be taken in the senior year. Each School has its own criteria for scores, which can be found in the Graduate Study in Psychology book in the Department Chair's Office.
GRE-ETS website: This has information about scheduling the GRE, as well as practice and diagnostic tests.
Recommendations. Letters of recommendation include the faculty member's opinion about your motivation, reliability, ability to work independently, and integrity, as well as his or her assessment of your academic abilities. Therefore, you should work individually with as many faculty members as appropriate, in Research Experience, Research Practicum, Internship, Independent Study, and Honors Project. You should also be aware that judgments are made about motivation and reliability based on behaviors such as regular class attendance, habitual tardiness, missing appointments, consistently failing to meet deadlines for papers, and classroom demeanor. Also see Reference Letter Checklist.
Relevant Experience. Master's programs look favorably on Internships and Research Experience. Doctoral programs require evidence of research ability, which can best be demonstrated by Research Practicum, Independent Study, and Honors Project. Presentation of an empirical study at a regional or national psychology conference would be make a candidate for graduate school stand out from other applicants.