When you cite a source, you are telling your reader "this is information is high quality. I am willing to trust this information and design an experiment - invest time and research funding - based on these data." You're also giving your reader an idea of what was going through your head when you were writing your paper. Since your paper for this class won't include everything on the topic, your sources are the way you inform your reader of the information you were working from.
Citations are the way your audience can follow in your footsteps, as it were. When done correctly, citations in articles hundreds of years old can cite sources hundreds of year older - and you, the reader, have enough information to find a copy. Please take a few moments to make sure your readers have complete and properly formatted citations to begin their search!
Not only do you need to cite your sources, but you need to format them according to a particular style. Your instructor will often ask you to use a certain style in your paper. For this course, you will use APA (American Psychological Association) format.
"Academic dishonesty is prohibited in all programs of the University. Academic dishonesty includes but is not limited to: cheating, fabrication, plagiarism, and facilitating dishonesty." University of Massachusetts Amherst Academic Honesty Policy and Appeal Procedure
Here are some links for more information about avoiding academic dishonesty: