Credit your source if you are using someone else's ideas in your paper.
Not only should you cite your source for quoted text, you must cite your source for ideas if the ideas are not original with you* and not common knowledge.**

Graphic by Sara LeBlanc
*As a rule, error on the side of caution and provide a citation if you have any doubt. When to provide credit can be a matter of judgment. Your sense of when a citation is needed will develop with experience.
** "Common knowledge" is information that is widely known and easily verifed in a standard reference work.
Next: Taking notes and citing sources.