Familiarize yourself with the call number system. This is how books are categorized and classified. The Dewey Decimal System has 10 main classes, organized by subject. The 10 classes range from .000 to .900. Call numbers are located in the spine of the book. Some examples of classes include .200, which is religion, and .800, which is literature.
Learn the divisions. Each of the 10 classes is divided into 10 divisions. For example, literature is .800. Literature books are categorized by specific topics ranging from .800 through .890. For example, some divisions of literature are .800 -- "Literature and Rhetoric," and .810 -- "American Literature in English."
Remember that you file by digit, not by whole number. For example, in the series of call numbers 451, 451.01, 451.2 and 451.3, you can see that 451.01 would come before 451.2, because 0 is smaller than 2. The smaller numbers come from the specific classifications of books.
Remember that the more numbers a book has, the more specific it is. For example, the 820s are English and Old English literature, which become more specific with each single digit. For example, 821 is English poetry, while 822 is English drama.
Learn cutter numbers. These appear under the call number, begin with the first letter of the author's last name, and are followed by a series of numbers that help maintain organization within a certain subject area. An example of a cutter number is W43K. In this case, the author's last name begins with a "W," and the "K" indicates that the title of this work begins with a "K." The "43" tells the librarian that this book was written by the 43rd author in that particular subject area. These are important when looking for authors who have published multiple books.