Most college libraries use the Library of Congress (LC) system to arrange their books. The main thing that you need to know about this system is that LC books are arranged alphabetically by subject; conveniently, these subjects usually go along with college majors. Here's a list of all of the subjects in LC - as you can see, Psychology books are in the B's along with Philosophy and Religion.

A call number is the code that you use to find a book on the shelf.
Tip: The most important thing to remember about Library of Congress is that books are arranged alphabetically by subject.