Keyword research is crucial for success with your online writing. You can pick up free information on Keyword Research from WordTracker here.
Brian Clark, of Copyblogger, has written a post on keyword reasearch that is excellent. He wrote it for bloggers, but the information is applicable to any online writing you are involved in. Check it out here. This is a 5 part comprehensive keyword research guide that shows you how to find your keywords and use them in your writing. This guide was put together by Brian and his associate editor, Jonathon Morrow.