Add a copyright notice to your game. This is not required, but it's probably a good idea. A copyright notice will stop an infringer from claiming that his infringement was innocent, and it reminds people who play the game that they shouldn't copy it without permission. A copyright notice consists of the word "Copyright" (or the "c in a circle" symbol), the year the game was first published, and the name of the copyright owner. You don't need to put it on each card; putting it on the game box is enough.
Determine which form to use. If your game has any writing -- the directions, for instance -- the Copyright Office suggests that you register it as a written work, using form TX. If your game consists solely of graphics, than it would be more appropriate to register it as a visual work using form VA.
Decide whether to register electronically. Electronic registration is generally faster and costs less, and you may be able to send electronic copies of your game instead of using the mail. To register online, go to the U.S. Copyright Office website, click on "Electronic Copyright Office" and follow the directions to set up an account. Otherwise, print out the appropriate form.
Fill out the registration form. The form will ask for information about the game (referred to as the "work") and its author or authors. If multiple people worked together on the game -- a writer and several artists, for example -- make sure to include them all as authors. If the forms confuse you, get help from a copyright attorney or an online legal document service.
Prepare a copy of the game. If your game has been published, you must send an actual copy of it to the Copyright Office. If it has not been published, you have the option of sending an actual copy or sending "identifying information." The identifying information usually consists of photos or drawings of the card game, and must include everything copyrightable. That means every single card if you have unique art on each card. One advantage of sending identifying information instead of an actual copy is that you can send it digitally if you register your game electronically.
Submit your application and deposit. Don't forget to include the appropriate filing fee. Once the Copyright Office processes your application -- which will take several months -- you will get a certificate of registration in the mail.