Simply head on over to our giveaway page, click the 'Claim your key' button at the bottom of the page and you're set with a shiny new key! From here, you can register for the beta on the preview page and follow this handy guide for complete information on downloading the client and joining the beta community. Best of luck!
Update: Champions Online is currently in a closed beta and, because of this, the servers are running in playtest sessions. Currently, the servers are down, so please don't panic when you download the client and it cannot connect to a server. Please refer to the Champions Online preview page for more information about the next playtest session.
Update 2: 50 minutes of giveaway, and all 1,000 codes have vanished! Congratulations to those of you who were lucky enough to snag a Champions key, and enjoy the beta!