The first step is to determine keywords that sum up the main concepts of your topic. Keywords are typically nouns or noun phrases. If you phrase your topic as a research question or thesis, you can often pull keywords from the topic sentence.
Example topic: What is the role of mass media in shaping public opinion?
Keywords:
Sometimes the keywords from your original topic sentence will not produce the types of results you want. When that happens, try to think of related keywords. These can be other words that have similar meanings, words that are broader (good for when you have too few results), or words that are more specific (good for when you have too many results).
Example related keywords:
Similar: internet, information media, mass communication
Broader: communication, media
Narrower: newspapers, blogs, television