When you cite you indicate that information is borrowed from a source. You need to do this every time you borrow information.
Why do you cite?
The MLA Handbook, 9th edition, is the ultimate guide for using this citation style. The St. Martin's Handbook and credible online resources for using MLA citations are all based on the MLA Handbook. The library has two copies; ask for them at the Library Services desk.
The Publication Manual of the American Psychological Association is the ultimate source for using this citation style.