You can often find citation information next to the title of an article in a database or in the "cite" tab below articles listed in Google Scholar. Watch out, however: these machines often make mistakes, so you still need to know how to write a citation!
Purdue University's Online Writing Lab (OWL) is a great resource. It gives detailed information about how to write citations with examples.
You can also find citation and style manuals in the reference collection on the first floor of the library.