Knowing how to cite is important, but let's start by discussing what to cite and why.
Here's the short formulation:
If you had to look it up, you have to cite it.
That means any direct quotation from a book (or movie or Web site or whatever), however short, should be cited.
It also means that any fact you've taken from your reading should be cited. The only exception is “common knowledge” — if something is widely known, you needn't say you read it in any particular source. So, for instance, if you want to point out in passing that February is the shortest month, there's no need to footnote it. It's usually not necessary to cite encyclopedias or dictionaries for something widely reported in other sources. But if you quote their exact words, cite them.
How do you know when something is “common knowledge”? As a rule of thumb, if you're in doubt, cite it. Few professors will object to too many citations, but we get grumpy when things that should be cited aren't.
That's what to cite. How about why? It's not just a matter of performing arbitrary gestures to keep your professors happy; citation is serious business. It amounts to putting your intellectual cards on the table. It allows a reader to check your facts, and to follow up your argument by returning to the sources you used. It also shows that you understand what you've read, and that you're not simply regurgitating what you've found.
Now, how do you cite something? There are several competing standards; your professors may prefer one over the others, but they're all equally “right.” Different disciplines (math, psychology, medicine) have their own styles, but there are two common ones in American English classes, Chicago style (named for the Chicago Manual of Style, now in its fourteenth edition) and MLA style (named for the Modern Language Association, which publishes the appropriate guidebook). If you're an English major, you should probably pony up the dough for the MLA guide.
There are two main places you can cite your sources: one is in footnotes (or endnotes); the other is in a list of works cited.
Here are some standard citation formats, as they appear in both Chicago and MLA. First a book, then a journal article, then a book article:
|Footnote||Tobias Smollett, The Expedition of Humphry Clinker, ed. Lewis M. Knapp (Oxford: Oxford Univ. Press, 1966).||Tobias Smollett, The Expedition of Humphry Clinker, edited by Lewis M. Knapp (Oxford: Oxford UP, 1966).||Works Cited||Smollett, Tobias. The Expedition of Humphry Clinker. Edited by Lewis M. Knapp. Oxford: Oxford Univ. Press, 1966.||Smollett, Tobias. The Expedition of Humphry Clinker. Edited by Lewis M. Knapp. Oxford: Oxford UP, 1966.|
|Articles in Periodicals|
|Footnote||Alan D. Hodder, “In the Glass of God's Word: Hooker's Pulpit Rhetoric and the Theater of Conversion,” New England Quarterly 66, no. 1 (Spring 1993): 67–109.||Alan D. Hodder, “In the Glass of God's Word: Hooker's Pulpit Rhetoric and the Theater of Conversion,” New England Quarterly 66.1 (1993): 67–109.||Works Cited||Hodder, Alan D. “In the Glass of God's Word: Hooker's Pulpit Rhetoric and the Theater of Conversion.” New England Quarterly 66, no. 1 (1993): 67–109.||Hodder, Alan D. “In the Glass of God's Word: Hooker's Pulpit Rhetoric and the Theater of Conversion.” New England Quarterly 66.1 (1993): 67–109.|
|Articles in Books|
|Footnote||B. Hall, “Erasmus: Biblical Scholar and Reformer,” in Erasmus, ed. T. A. Dorey (London: Routledge & Kegan Paul, 1970), pp. 81–113.||B. Hall, “Erasmus: Biblical Scholar and Reformer,” in Erasmus, ed. T. A. Dorey (London: Routledge & Kegan Paul, 1970), pp. 81–113.||Works Cited||Hall, B. “Erasmus: Biblical Scholar and Reformer.” In Erasmus, ed. T. A. Dorey, pp. 81–113. London: Routledge & Kegan Paul, 1970.||Hall, B. “Erasmus: Biblical Scholar and Reformer.” Erasmus, ed. T. A. Dorey (London: Routledge & Kegan Paul, 1970), 81–113.|
One thing you don't usually have to cite with similar precision is a reference book. If you want to quote the Oxford English Dictionary, for instance, you needn't spell out the volume and page numbers; ditto for encyclopedias. (It's understood that you found the information on the word mulct under the word mulct.) If, though, you discover something in a different entry from the obvious one — if, for instance, you find information about Einstein in the encyclopedia not under “Einstein, Albert,” but in the general entry on “Physics” — you can use “s.v.” (Latin sub verbo, “under the word”) to indicate the source: “See Encyclopædia Britannica, s.v. ‘Physics.’”
There are plenty of difficult cases, and I haven't the time to explain them here. How, for instance, do you cite episodes of television programs? — government documents? — corporate annual reports? — interviews you conduct? — information gleaned in an on-line chat? Standards for most of these are in the major style guides (Chicago and MLA), though the Internet presents a moving target for bibliographers.
Teachers will vary, but as a rule, I don't worry too much about getting such tricky cases “right.” After all, there isn't necessarily a single “right.” All I need is a good-faith effort to document the source of the information. That will usually include the author's name (if it's known), the title of the work, the larger work of which it's a part, the date it appeared, and the publisher — whatever applies.