Once you've settled on a research question, you'll need to develop a search strategy to start applying to the databases and other resources you use. If you don't have an effective search strategy, you will not get good results from your searches and may miss important research that should be in your review.
One way to start is to break down your research question into its core components and create a list of keywords that are related to, synonymous, or are narrower or broader versions of your initial terms. You can see an example below.
These keywords can be combined using Boolean operators. Boolean operators are words (AND, OR, NOT) that can be used in combination with your keywords to produce more relevant search results. For example, you might combine 19th century with Civil War. 19th century AND Civil War will give you results that include both of those keywords. 19th century OR Civil War will give you results that include either of those keywords, but not necessarily both. 19th century NOT Civil War will give you only results that mention the 19th century, but throw out the results that mention the Civil War.
When you get too many, too few, or irrelevant results, you can use your keyword bank to try different keywords and keyword combinations to retrieve different results. The literature review process is not a one-and-done, search only once deal, so you'll want to have multiple search strings you can apply to different resources in order to capture all of the research you might want to include. Consider also the kinds of language used in different sources - in order to find the information, you have to use the language the authors would use. Scholarly articles will use more formal, technical language than a newspaper article would, for example.
You may also want to consider searching by subject. Keywords identify any instance of a word or phrase being used in the text, but subjects describe the focus of the work. You can find some example subjects for your searches through the Library of Congress Subject Headings page.