Am I allowed to make my game before the jam starts, I really don't know

I don’t think that’s allowed. Besides, the theme is announced the day the jam starts, so it’s best to wait even for your own sake

No, you cannot have a pre-made game but u can have pre-made assets for the jam.

Example :- scripts, art, music etc.

This is my first jam and theres no way your allowed to make a game before the jam starts. But you could get some pre game stuff ready like music / assets or find someone to collab with. and there is a theme, im pretty sure you get a voting period for the theme like a couple days before it starts. (once again i dont know, this is my first game jam)