I am a copy editor professionally, and I'd be happy to help anyone with specific questions. To answer the original question, this is an easy-to-use Web site: http://ccc.commnet.edu/grammar/

I find that the online version of the American Heritage Book of English Usage is helpful, too: http://www.bartleby.com/64/

Originally posted by Karen
Any specific questions, just ask me! I'm an editor/proofreader (as well as Art Director) at my "other" job. And if I ever am a gazillionaire, I will institute mandatory attendance, nationwide apostrophe seminars. . .
Karen, I know just how you feel. However, my mandatory seminar would be about intrusive commas.