Well, call me someone who is unoriginal and doesn't care, but I'd go, oh... five years back. High school. I know where I'd be, and I'd walk in, grab my 14-year-old self, and say, "We need to have a chat."
The next hour would be me telling my younger, stupider self to work harder at debate, work harder at theatre, and that guy? The football player across the room in English? Talk to him. Now. Ignore all the other male specimens in the vicinity, because he's going to be the only one who gets you AND puts up with your shit. Also, [insert-everything-else-here].
Of course, those things not being an option... I wanna go back to New Mexico when Billy the Kid was running around, and pop Pat Garrett in the mouth before he gets a chance to kill the guy.