Formal:
* Que le plaisir commence! (This is the most direct and literal translation)
* Que la fête commence! (This emphasizes the celebratory aspect of the fun)
Informal:
* Que la fête commence! (This is also common in informal settings)
* On y va! (This means "Let's go!" and implies getting started with the fun)
* C'est parti! (This means "Here we go!" and implies excitement and readiness for the fun)
The best translation depends on your audience and the specific situation.