The Library of Congress Subject Headings (LCSH) uses what is called controlled vocabulary. They are the big red books kept behind the reference desk. You can also see them at depending on your major and your future vocation, you might actually work with a database which has a controlled vocabulary. You will learn what the appropriate search term is and how to narrow it or broaden it. Usually, you get what is called a "scope note" which describes the topic. There are a few terms you will need learn: UF means "Used For" and it is not recommended that you use the terms in this list. "BT" means "broad terms" and these terms are broader than the term you are looking up. The opposite is true of "NT" or narrow terms. These are more narrow than your term. "RT" means related terms which are synonyms. This is useful for other things as well. LCSH is also a great source if you need additional terms for database searching outside the catalog.
Here's what a page looks like.
Now you can view them online at: http://id.loc.gov/authorities/search/. This site now allows you to view the book online and see broader, narrower topics. I think it lacks something by not including as many RTs (related terms). The site mostly includes BT (broad) and NT (narrow). It does have a very cool visualization tab to see the relationship of the terms. This works only with terms which have no more than twenty relationships.
After determining your thesis, choose a database or web site appropriate for the subject being searched. Use the synonyms and variant spelling terms (from above) in your search. Read the help screens for that database. Review the library’s database and web site listings and list the resources you think will best help you on this assignment.
You can also use subject headings to get additional ideas for search terms for use with database searching.