Start with 2-3 important words relating to your topic. If you start with too many, it may limit your search too much and you may miss resources that would be perfect for your paper.
Add on extra words to narrow down your results. More words = fewer results.
Use "quotation marks" to keep your keywords side-by-side as an exact phrase, for example, "social mobility" or "american dream."
Use the * asterisk as a wild card to give you the same stem with any ending, for example, immigra* will give you the words immigrant, immigration and any other words that have the root immigra.
Use (parentheses) to nest, or combine similar keywords with the connectors OR, AND, NOT, for example, ("social mobility OR "american dream" or "upward mobility") AND (ethnicity OR education OR gender)