Now that you have a research question or your main argument chosen, write it out as a sentence.
Example: Do teens have a problem with drugs?
What are the main keywords in this sentence? The answer is "teens" and "drugs." We don't need to use the word "problem" here. We're going to search for "teens" and "drugs" and let the research guide us to the question's answer.
However, we may find as we search that the search terms we're using aren't retrieving the results we want. It is a good idea to come up with synonyms for your keywords or different ways of phrasing your question. Some fields of study use their own specific language to refer to various terms.
So for example: What are other words we can use for teens?
teenagers, youth, young adults, adolescents, children
What about drugs? How else can we refer to a "problem with drugs?"
substance abuse, drug abuse, drug dependence, chemical abuse, chemical dependence, addiction, and if you want to be more specific or alter your topic slightly: cocaine, marijuana, alcohol abuse, opioids, heroin, etc.
Make sure you keep track of which keywords you have used and which ones have worked for you and which haven't.
Now that you have your keywords, you are ready to start searching!