Citation management tools help you manage your research, collect and cite sources, organize and store your PDFs, and create bibliographies in a variety of citation styles. Each one has its strengths and weaknesses, but all are easier than doing it by hand!
Zotero: Free software that keeps copies of what you find on the web, permits tagging, notation, full text searching of your library of resources, works with Word, and has a free web backup service. Zotero is available as a stand-alone application that syncs with Chrome and Safari, or as a bookmarklet for mobile browsers.
RefWorks - web-based and free for UC Berkeley users. It allows you to create your own database by importing references and using them for footnotes and bibliographies, then works with Word to help you format references and a bibliography for your paper. Use the RefWorks New User Form to sign up.