If you have chosen a topic, you may break the topic down into a few main concepts and then list and/or define key terms related to that concept. If you have performed some background searching, you can include some of the words that were used to describe your topic.
For example, if your topic deals with the relationship between teenage smoking and advertising in the United States, the following key terms may apply:
smoking -- tobacco -- nicotine -- cigarettes
teenage -- adolescents -- children -- teens -- youth
advertising -- marketing -- media -- commercials -- TV -- billboards
When listing the key terms or concepts of your topic, be sure to consider synonyms for these terms as well. Since research is an iterative process, you will also find additional key terms to utilize through the resources you encounter throughout your research process.