Use AND to combine keywords for more specific searches, example: environment AND cancer
Use OR when there's multiple ways of saying the same thing, example: mental health OR mental illness
If there are multiple endings to a root word that are all relevant to your search, you can cut off the part that differs and replace it with an asterisk. Example: environment* will find "environmental", "environment," "environmentalist," etc.
If keyword searching is not yielding good results, try limiting your search to just sources that have your keyword in the title.
When looking for books in WorldCat, consider using Library of Congress Subject Headings. These are words human indexers assign to books to describe the main content and therefore can sometimes be more precise. Articles are not assigned Library of Congress Subject Headings.
Once you find a good resource, you can use it to find more by looking at:
Subject headings (in databases that have them like WorldCat & PsycINFO)
Looking at the keywords the authors used
Sources cited in the bibliography
Sources that cited the source you have (which you can find through Google Scholar