I sent an itunes gift to an incorrect email address. How do I cancel it and get my money back

You can just resend the gift to the correct address:

http://support.apple.com/kb/HT1541

This is assuming, of course, that the gift hasn't been redeemed by the unintended recipient. If it has been, then there's nothing you can do other than re-purchase the gift. The iTunes Store will not be able to cancel the gift in that case and refund your money.

Regards.