"When do I cite a source?"
Whenever you include a word, phrase, or idea from a source, it needs to be cited. That source can include a web page, classroom lecture, an interview with your Uncle Pete, quote or summary from a book, magazine, etc. And note that I wrote "a" word — singular. Even a single word from someone else, when included in your own essay, needs to be set off with quotation marks and then cited.
"Which Citation Method Should I Use?"
First, you need to ask your instructor which citation method is used in your class. The two most common citation styles are Modern Langague Association, or MLA (used in English, History, Art, and other Humanities courses) and American Psychology Association or APA (used in Psychology, Sociology, Math, Biology and other Science courses).
"How Do I Cite Sources?"
For more complete information on these and other citation methods, go to Purdue's Online Writing lab.
More generally, it's important to learn the distinctions between summary, paraphrase, and direct quote.
"My Instructor warned us against 'Plagiarism' - what's that and how do I avoid it?"
Let's start with a basic definition: Plagiarism means taking words or ideas from a source without documenting them.
Words: any words (even one or two) taken from a source and included in your own work need to be in quotation marks and documented. There may exceptions to this (common knowledge), but when in doubt, use quotation marks and document or check with your instructor.
Ideas: when you get an idea from source, be it from an article, lecture, person, etc., it must be documented – even if you put it in your own words (and if you use the same words, they must be in quotation marks).
Source: the person, book, article from which you obtained information.
See the Plagiarism page on this site to see how to Avoid Plagiarism.