It is very important when writing papers to cite your source! Every semester students are caught plagiarizing papers. Many times it is a matter of ignorance, of not knowing how or when to cite the idea or passage. When in doubt, cite! Be sure and check with your professor for the preferred method of citation documentation and for help clarifying how to cite information.
The University of Michigan's policy on academic integrity is very clear about what the expectations are on campus regarding plagiarism. Please read the site so that you are familiar with the policies.
Miguel Roig, has written an extensive website, Avoiding plagiarism, self-plagiarism, and other questionable writing practices: A guide to ethical writing. If you have questions regarding your citations (or lack thereof) consult his chapter on The Lesser Crimes of Writing. And remember, your best source of guidance will be your professor!
Writing Guide for Philosophy from George Mason University (including Bekker and Stephanus numbering)
Chicago Manual of Style (from Ohio State University)
Turabian Citation Guide, 1996 (from Ohio State University)
Citing Government Documents (from Columbia University)
MLA (from Cornell University)