A "citation" is a reference to one publication. Often called a "reference." References are included in a a Bibliography, or Literature Cited, or References section of a paper.
Ask yourself the following questions:
Do you have a citation, and just need to find the article online?
Use the Finding Full Text Guide Research Guide
Do you have a citation, and need to put it in a particular format?
Use one of the following Guides:
APA Style Guide
MLA Format Guide
Ecology Format Guide
Chicago Manual of Style Format Guide
Do you have a citation and want to find other publications that list your citation in the bibliography?
Keep reading; you want to do a "Cited Reference Search"
This 3 minute video, from the University of Alberta Library does a nice job illustrating the concept of Citation Searching.
Go to the "Cited Reference Databases" in the menu to continue.