Make a girl laugh?
The best way to make a girl laugh is by making conversation then make a funny comment on what you two are talking about. Don't make it obvious that you are trying to be funny. Make it seem like you are naturally funny. Girls get bored if you try to hard to be funny.
Making a joke about the topic that you are talking about. It all depends on how comfortable you are with each other. If you know each other well, like make fun of something she does or something stupid that she says, but don't go too far.