You'll find your 12 digit code on specially marked 330ml cans, 500ml and 2l bottles of 'Coke', 'Diet Coke' and 'Coke Zero'. On can multipacks it is printed on the inside of the cardboard pack.
Next time you run out of stuff to talk about on a long, boring journey, you and a friend can both listen to your mp3 player in glorious stereo - without fighting over one set of headphones.