University of Texas Libraries provides a helpful definition of plagiarism.
When you conduct research, you gain an understanding of a topic by becoming familiar with the work of other scholars in the field. To make a valid scholarly contribution, you need to gather background information, look up original sources, and read previous studies. Only after you have done all this work, can you begin to formulate your own ideas on the subject.
You should also familiarize yourself with the John Jay College Policy on Academic Integrity. The college defines plagiarism as “the act of presenting another person’s ideas, research or writings as your own.”
University of Texas Libraries put together a quiz that teaches you to recognize instances of plagiarism.
Also from University of Texas Libraries, a quick guide to the kinds of information and sources you need to document.