Start the brainstorming! Now that you have a topic, you need to develop keywords and phrases that describe its core concepts/ideas. Use these keywords and phrases in your search to help focus it and produce more relevant results. Here is an example:
How does junk food marketing impact children?
Find core idea(s)/concept(s) > junk food, marketing, children
Other related keywords and phrases > advertising, fast food, youth, diet, unhealthy food, kids, food preferences
Next, put those keywords to work! Some library search tools along with Google automatically connect multiple keywords with "and." This narrows the results to include all of the keywords used in the search. Depending on the kind of information you need, you might use "or" and "not" to connect certain keywords.