Purdue University created the Online Writing Lab (OWL) to offer writing resources and materials at no cost. While this resource was created by and is maintained by Purdue, it is a comprehensive resource that is available to everyone.
OWL offers detailed instructions with examples for most citation formats, including APA, MLA, and Chicago. They've also included an online citation tool to help you create your citations in the correct format. Click the links below to visit the pages of each citation style.
The Writing Center: University of North Carolina at Chapel Hill
Scientific Writing Guide: Duke University Medical Center Library & Archives
The Writing Studio: Colorado State University