Ask them what they did to prepare for graduate school. They are a great source of information. The faculty in the Psychology Department are not only excellent teachers, they are also leaders in their respective areas of research. You should make every effort to get to know them outside of the classroom. Talk to them about different graduate programs, about the different areas of research, about what they do and how they decided what path to take.
Most graduate schools require three letters of recommendation.
These should be from psychology professors who know you well. The most valuable letters come from professors whom you have worked with on a research project or a supervised field placement. Prepare a resume that describes your interests, goals, educational experiences, special skills, and extracurricular activities. Meet and discuss the information on your resume. Bring your resume and addressed, stamped envelopes for the schools to which you have applied.