Your first step is to learn the basics about your psychologist: when they lived, what their major contributions were, and how they fit into the history of psychology. Don't underestimate this step. Carefully reading a few good biographical articles will go a long way in helping you perform your role.
Guess what? You can use Wikipedia! Although Wikipedia is a risky source since anyone can edit it at any time, it is a very helpful starting point for learning background information and putting facts in context. The References section at the bottom of any Wikipedia article will point you to published information.
Along with Wikipedia, you should find at least one other biographical article in a published reference work. It can be either electronic or print. Search for your psychologist by name using these resources:
One useful book in the print reference section is the Biographical Dictionary of Psychology. Go to the reference section (main floor of the library) to find the dictionary, then look for the call number (REF BF109.A1 Z85). To see whether your psychologist is included, look in the index at the back of the dictionary.