Common Sense dictates that whenever you write something in a paper that is not your own original thought, you should give credit to the person or group who gave you the words or idea.
Style gives you a framework that is consistent throughtout your project within which to show readers where you got the quote or idea and allows people to look at the source you used themselves.
In general, every element in your text that is not original needs a brief in text or footnote citation that corresponds to a full citation that appears in a list of references.
Use your own Common Sense and use the Style that your professor (or publisher) asks you to.
When you cite your sources you:
There are quite a few library research guides listed on the libguides research page
A couple of highlights from librarians' libguide are
APA & MLA Citation Game (really..it's kind of fun)