While the tuning that
fricto suggests is the most common tuning for a twelve string, it is by no means the only. Many 12-string guitarists take great
masochistic pleasure in
tuning to all kinds of odd things (each string up a fifth, or a third from it's six string cousin).
Also older twelve strings are often tuned to down a whole step (thickest string is D rather than E). The tension caused by all twelve strings in standard tuning used to snap guitars in half. Apparently that's not an issue anymore.