Any information that you took from another source needs to be cited, otherwise it is plagiarism. In general, your citation should be in the order of: Author, Year, Title, and Source.
References to published work go in two places in your essay.
1) First, you need to include a citation in the body of your essay. These should almost always go at the end of a sentence and include a Name and Year in parentheses.
If there are two authors: ...(Smith and Cardon, 1992).
If more than two authors, use: (Smith et al., 1993). Note: et al. is Latin for et alii , meaning "and others". Because it is Latin, you need to underline or italicize the words. Note that et is not an abbreviation, whereas alii is abbreviated (hence the period).
2) The complete references should be placed in a section entitled "Literature Cited" or "References" at the end of your essay.
The appropriate format for a book is:
Starr, C. 2006. Biology: Concepts and Applications. Sixth Edition. Thomson Brooks/Cole Publishers, Belmont CA, USA. Pp. 624-625.
Note that the title of the book can be italicized or underlined. Also include the associated page or pages.
The appropriate format for a journal article is:
Smith, J. 1994. Something I saw in the field this summer. Journal of Really Neat Findings 35: 135-137.
Note that the title of the journal should be italicized or underlined, but the title is neither underlined nor italicized.
The appropriate format for the Biology 102 Lab Manual is:
Department of Ecology and Evolutionary Biology, University of Connecticut. 2007. Biology 102 Laboratory Manual, Fall 2007 edition, University of Connecticut, Storrs, Connecticut.
The appropriate format for a WWW article is:
Wikipedia contributors. Vitamin C [Internet]. Wikipedia, The Free Encyclopedia; 2010 Sep 20, 16:44 UTC [cited 2010 Sep 20]. Available from: http://en.wikipedia.org/w/index.php?title=Vitamin_C&oldid=385937295 < http://en.wikipedia.org/w/index.php?title=Vitamin_C&oldid=385937295 > .
Do the best you can here as web pages will likely have many different formats (i.e, some will have an author indicated, some will not). Make sure to include the web address.