Refine your topic: an overly-broad search will return an unmanageable number of results, while a topic that is too narrow won't produce enough information. Play around with your search terms until you find a topic that is researchable.
Choose keywords: once you have chosen a topic, you need to identify key words that you can use to search databases. Do some research to find out if there is "official" terminology related to your topic. You may have to try a number of synonyms until you find the one that is best for your search. Tip: some databases also use specific terms called descriptors/controlled vocabulary to organize articles. Knowing the descriptors for your topic makes it easier to find relevant articles.
Combine terms: once you identify the best terms for your topic, put them together with Boolean operators such as AND (for more precision in your search), OR (to enlarge the pool of results), or NOT (to exclude possible elements unrelated to your topic). These techniques work in most databases, as well as in Google.