I don’t know much about copyright and what I do know is what I think I know. =p
As soon as you write something on any tangible medium (paper, web, napkin) it is copyrighted without the need to use the circle C (©) or provide any form of copyright notice. Most countries operate in this manner and copyright lasts your lifetime plus a number of years. In most countries, it lasts 70 years after your death.
Games are odd in that game mechanics can’t be copyrighted.
You can make your own version of Catan, say Mint Tin Matan, and exactly copy the game mechanics. BUT . . . you can’t copy the art or instructions verbatim.
Copyright protects your instructions and your images. You don’t have to do anything to invoke that copyright protection . . . in theory.
“Publishing” your game, even just one play test version printed at home and stuffed into some box is enough to claim copyright.
However, the challenge comes in proving when you copyrighted your game. The poor man’s copyright of mailing it to yourself doesn’t hold much water because it doesn’t prove you created it.
If you’re ever challenged over your copyright, you may need to legally defend it. If you ever want to go after someone who completely stole your work, proving it’s your copyright may require a legal court case. Going to court costs big bucks.
In the US, and many countries, you can formally register your copyright which makes your claim much stronger in court. A registered copyright is considered prima facie in court and that means you’ve met the burden of proof that it’s yours.
In the US it costs $35 and can be done almost entirely online here. While it doesn’t stop anyone from copying your game, it might give you a little peace of mind, plus you’ll send a copy of your game to the Library of Congress. Kind of neat to think your game will be forever archived. =)
In lieu of a formal registered copyright, adding your game to the BoardGameGeek database would be compelling in proving it’s your game.
Mint Tin Cancarssonne anyone? =D