I'd get pissed off at first, knowing i'd only have lived for 13 years (nearly 14) out of a life that may have become 70-80 years long. -_-
Then, i'd probably start not believing it.
BUT, if I got proof that seemed completely true, i'd most likely lay in my room and cry for several hours. After that I would most likely tell each of my friends (internet friends too) that I love them all.
I'd confess a few things that shouldn't be confessed here.
I'd not steal anything. I'd rather buy a lot of stuff that's bad for you and enjoy the last day of my life.
Oh, and i'd try smoking. Just to see what it's like. It wouldn't hurt me when it's the last day of my life anyway. Not like i'm ever going to try it, unless something like this happens. :P
The very last minutes of my life i'd spend telling my family how much I love them.