Formal:
* Mujhe hasi aa rahi hai. (This is the most direct and literal translation.)
* Main hasne laga/lagi hu. (This means "I am starting to laugh".)
Informal:
* Mujhe hasi aati hai. (This is a more casual way to say it.)
* Mujhe hasi aarahi hai. (This is a more informal version of the formal translation.)
* Ha ha ha, mujhe hasi aa rahi hai! (This is a more expressive way to say it.)
You can also use slang like "Haan, maza aa gaya!" (This means "Yes, I'm enjoying this!" and can imply that something is funny.)
Remember, the best way to say something depends on the situation and who you're talking to.