In the process of writing a paper, you are going to use the knowledge and works of other writers and researchers. Whenever you do that you will need to document your source by indicating what you borrowed and from whom. There are various styles of citing documentation:
APA Style – American Psychological Association style - psychology, education, and other social sciences.
Chicago Style – The Chicago Manual of Style - used with all subjects in the "real world" by books, magazines, newspapers, and other non-scholarly publication.
MLA Style – Modern Languages Association style - literature, arts, and humanities.
Mendeley - Mendeley is a free reference manager and PDF reader designed for faculty and students for managing and sharing research papers, discovering research data and collaborating online.
NoodleTools Express - Online tool for MLA, APA and Chicago Bibliographic Formats. It takes care of punctuation, alphabetization and formatting, producing a polished source list for import into Word.
Zotero is free tool that collects, manages, and cites research resources. Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies.