I’m one of those odd people who actually enjoys crawling all over the U.S. Copyright Office’s website, looking for that little bit of information that I want. However, I know I’m in the minority, so I’d like to point you to one of the best introductions to copyright that I’ve run across: The Copyright Handbook: What Every Writer Needs to Know. The author, Stephen Fishman, cuts through a lot of the difficult material to present you with the facts about copyright that you’ll need to protect your work.
However, I’d like to warn you that if you get into a knock down drag out fight with someone about copyright, get the advice of a copyright attorney before you have to go into court. It’s a legal issue, so don’t fight without protection.