Make sure that you cite your sources correctly to avoid plagiarism. See the following resources to better understand how to prevent plagiarism in your writing.
Three Basic Varieties of Plagiarism
Using another's words without giving credit.
Using another's ideas -- paraphrasing or simply changing words around -- without giving credit.
Citing sources that weren't used in the paper, or making up references.
Avoid PlagiarismSection from Plough Library's How to Research a Topic guide.
Excellent tutorial from Acadia University for students, with specific examples of how to quote and paraphrase correctly, as well as instructions on source citation and good research habits.