Where to start

Strunk and White's The Elements of Style is a good starting point.

Using an online writer's reference

Other writer's references such as Diana Hacker's A Writer's Reference, the Holt Handbook, and the Bedford Researcher have sections that address punctuation. Some instructors prefer one of these guides instead of or in addition to a formal style manual, and some professors use the rules of a particular writer's guide as a grading standard. Check with your professor to find out what you need to do for a partiuclar assignment.

Use the OWL at Purdue

To print out tip sheets and handouts about grammar, punctuation, and usage, make sure to visit Purdue's OWL archives.