If you are searching for a phrase, such as spontaneous combustion or greater rice weevil, put the entire phrase in double quotes. This will tell the search engine to only find results that contain the exact phrase, rather than one or two of the individual words.
If you have two or more words or concepts that you want to find, use AND (must be capitalized) to tell the search engine to only look for items that contain both words.
Often, there is more than one way to talk about your topic. For example, if you are looking at the study habits of college students, you could look for "study strategies," "study habits," "ways of studying," "study methods" etc. Some phrases might be better than others. If you want to try looking for multiple variations of the same word or phrase in a single search, use OR (all capitals) to tell the search engine to find material with any of the words you've included.
You can even get fancy and use both AND and OR:
Use this box to look for additional databases by subject. You can also use the A-Z database list to browse.