Your research project should build on a well-defined and well-studied area of research. Developing and focusing your research hypothesis will make putting together your research proposal and project much easier.
Do some exploratory research on your broad research idea in your course textbook, class notes, and PsycINFO to identify more specific issues and arguments in your research area and possible relationships between them. You should also identify the methodologies and tests that are used to study your research area, as well as the populations that are studied.
Ask yourself questions about your research topic: What interests me about this topic? What have people said about it? What gaps, contradictions, or concerns arise as you learn more about it? What relationships are there between different aspects of the topic?
Write a research question that your hypothesis answers: Use the information from your exploratory research and your answers to questions about your broad topic and the area you've decided to explore to build a focused, clear, simple research question
Identify the key concepts of your research question: what concepts will you need to define and measure in a study to answer your research question? How will you operationally define these concepts into numbers that you can analyze?
Identify your variables: Use your operational definitions to identify and list the independent and dependent variables for your research question. Identify possible confounding variables and the variables you would use to control for them.
Choose a current topic: Develop a hypothesis for a research area about which articles are continuing to be published. Avoid defunct or little-known areas of research.
Write about what interests you: Professors want students to develop experiments in areas that they care about. If you're interested in the topic, it will be more fun for you to do your experiment and write up your research paper, and probably more fun for your professor to read it, too.
Ask your professor for feedback on whether the hypothesis you develop is a good hypothesis, one that can be tested.