"crack someone up" means "make someone laugh" If you were to tell someone a joke, for example, they could reply "you crack me up!" instead of saying "you make me laugh!"

There are actually two meanings for "crack up", but the more common one is about making someone laugh. "You cracked me up." The other meaning has the opposite emotion! You start to cry, but no-one does this to you. It's your own action. "I cracked up." I'm not sure if the second meaning is typically Australian, but I recognise it as a common expression.

It means "making me laugh".. for example: This comedian is really funny, he cracks me up