When you look at a book's record in the online catalogue, it displays the call number on one line like so:
When you look for a book on the shelf, the call number will be arranged vertically on the spine of the book, usually divided into four lines:
Read the call number from top to bottom, left to right, line by line.
First line is one or two letters, read alphabetically. Second line is a whole number (from 1 to 9999) Third line is a decimal number, not a whole number (i.e. K89 comes before K9)
The fourth line may be either another decimal number, or it may be the date that the book was published.
The following video (by Douglas College Library) will teach you how to read call numbers:
Other types of call numbers
There are a few special collections in the library that use different types of call numbers. These can be searched by using the Local Call Number option in the Catalogue search.