When choosing your own topic, begin by considering topics covered in your course and textbooks/readings that fit the assignment. Then do some background research (covered on next page) on one or more of those topics to get a better understanding. This will not only help you narrow your focus, but equip you with the necessary vocabulary (names of people, places and things related to the topic to search scholarly literature.)
A subject is broad and general, while a topic is a specific issue relating to the subject. For instance, climate change is a subject. "What is the effect of global warming on the salinity of the oceans?" or "What is the effect of ocean salinity on marine life?" is a topic.
Research is a form of problem solving, and the first step in problem solving is always to identify the problem. In other words, what question do you hope to answer with your research?
Consider the who, what, when, where, why and how to help you develop your topic.