When using the exact words of another author always use quotation marks and a proper citation. Make sure to use the citation format that your professor requires, which might be different for different classes and disciplines.
The library keeps several style guides on reserve at the circulation desk for use in the library, including the MLA Handbook, Publication Manual of the American Psychological Association, and Turabian’s Manual for Writers of Research Papers, Theses, and Dissertations. For quick reference to all of these styles you might try A Writer’s Reference by Diana Hacker, also on reserve at the circulation desk.
Even when not using a direct quote, or paraphrasing, from another source you might need to provide a citation. Paraphrasing is not just replacing the words of the original source with synonyms, and is best done without looking at the source.
There are exceptions, for example, when something is “common knowledge”. Facts can be considered common knowledge when they are repeated over and over through multiple sources, or if it is something you can safely assume that your readers will know. For example, the fact that George Washington was the first President of the United States is common knowledge.
But when in doubt, cite.