Plagiarism occurs when you use someone else's ideas without giving them due credit. You can use other people's ideas provided you acknowledge where you got your information.
It is very important to cite the information that you use for your papers. As an author you are responsible for the references you include in your paper. Citing information accurately not only allows your professor to locate the information you have included; it also adds to your credibility as a researcher and writer. Above all, it helps to avoid plagiarism which is a serious offense.
Check Purdue's Online Writing Lab (OWL) for additional help.