Brainstorm topics that you find interesting. If you are already in a psychology class your instructor may give you a choice of topics from content that you have covered. In this case, make an organized list of topics and considered which ones pique your interest. You can begin narrowing your topic from this list. Some topics you might consider include theories of developmental, cognitive or organizational psychology; the history and ethics of experimental research; theories of the mind and applications of modern behaviorism.
If you have a topic and are unsure if it is acceptable, discuss your ideas with your instructor. It is her job to help you with these types of problems. She can also let you know what she is expecting from the paper, which will likely help you to narrow the topic.