You must cite your source of information any time you use another person’s ideas, opinion or theory. You must also provide citations for any facts, statistics, graphs, or drawings that are not common knowledge. Quotations of another person's actual spoken or written word and paraphrases of another person's spoken or written words must also be cited.
The Library has some great resources to help you become more familiar with new citation styles, on our Citation Guides page:
MLA (Modern Language Association) Style