The librarians strongly recommend that you develop a vocabulary that contains words describing your topic. This should include synonyms for all your key concepts. If the computer finds nothing using one word, try doing the same search again but substitute a synonymous term. You may get quite a different result from the machine. In addition to having your vocabulary prepared, you should already have decided how you want to combine your keywords, i.e. which operators you want to use with which words.
Computers are very literal-minded and very unforgiving.
If you don't know what you want to look for and how you want to look for it, the computer will probably be of little use. You could easily spend a great deal of time searching around and find nothing. You may use more than one operator in a single search. This search would look for records that have all three words. This search would look for records that have any of the three words.