I'd steal a couple guns, ammo for each, then go kill Hitler, the guy who killed John Lennon, I'd make sure Reagan had died, cause I wanna assassinate someone with my time powers, and I wanna cause the medical examiners to go "WTF?!".
I'd also stop 9/11, Bush from ever being elected (either time), and all other tragedies.
Oh, and King Leonidas would win, if I had my way, cause the Spartans would march into battle with GE-134 Electric Boogaloo Chain Guns, with underslung M79s.
Yeah...