Both of these tools provide online guidance for the major citation styles (APA, MLA, Chicago, and Turabian), with examples for both print and online sources:
Citing Sources From Duke University Library
Research and Documentation Online by Diana Hacker
These handouts were developed by the Parkland Library Staff and the Writing Center:
This website will put citations together for you. You fill in the blanks and it does the rest!
WARNING: Double check it's accuracy. Anything that does the work for you will have mistakes. Nothing is perfect.
This service will do both APA and MLA citations.