These sites will help you understand plagiarism and how to avoid it:
Understanding and Preventing Plagiarism - from Accredited Schools Online
10 Types of Plagiarism - video from Turnitin.com
Avoiding Plagiarism - tips from Hamilton College, NY
The DBU Writing Center exists to help students write better papers. Their web site explains the services they provide, and includes great handouts (available online as PDFs) that will help you learn the ins and outs of APA, MLA, and Turabian styles.
Other sites that offer great information about citation styles:
Purdue University OWL - (Online Writing Lab)
Writing Rescue Guide for College Students - style guides, tips, and expert advice
Official Sites with links to the parent organizations, including purchase information for the official style handbooks:
ZoteroBib helps you build a bibliography instantly from any computer or device, without creating an account or installing any software. You can seamlessly add sources and produce perfect bibliographies.
Zotero - free!
Mendeley - free!
EazyPaper - commercial
EndNote - commercial
RefWorks - commercial