Give credit where credit is due. By properly citing the sources you use in your research projects you are both identifying the resources that you used to complete your work and you are formally acknowledging the authors or creators of those resources.
You'll find many references in the course of your research. Make sure you keep track of what you find! Citation Management Systems are excellent tools to organize your references.
A Manual for Writers of Research Papers, Theses, and Dissertations, Eighth Edition by 
MLA Handbook by 
Publication Manual of the American Psychological Association by 
The Chicago Manual of Style by