your, you're

'Your' is the possessive form of 'you.' He's wearing your shirt. Punch him in the teeth and take it back before he stretches it with his fat no-neck.
'You're' is a conjunction of 'you' and 'are.'

Storm Shadow shook his head at his new ninja-in-training. "You're going to hurt your fist if you keep punching like a moron."