Citation tools can help you manage the research process by tracking what you've read and creating and formatting your bibliographies.
Wondering which one might be best for you? See How to Choose: Zotero, Mendeley, or EndNote.
For assistance with citation styles such as APA and MLA, please see Citing Your Sources & Writing Styles.
Another option is EndNote, which comes in two flavors: EndNoteWeb, which is free to you while you are at Washington University (but not available to alumni), and EndNote desktop software, which you can purchase at an educational discount at the Campus Bookstore. More info.