I'm going to make a joke to my friend.
What kind of joke are you going to make?
I couldn't help but laugh at him falling down on the floor
Technically, you shouldn't make fun of him at his expense but if he is a close friend and pretty down to earth, it is probably ok. Just be sure to not cross the line with the jokes or he might get mad at you.
Hehe I like to make fun of him for whatever he does.
Well like I said, if you aren't hurting his feelings and he's laughing along with you then it's fine.