If you already got hired, I would just relax. Get some exercise, find a couple good books [1] you can read later when you want to take your mind off work. Then, when you start work, pour over the source code. Understand it. Ask or research anything you don't understand. Make a list of things that suck about it -- there likely will be a lot of technical debt, but who knows... If you want to get ahead, combine working…
Gonna second this. Do something fun , whatever it is. You don’t need to be a hero the first day. Go in well-rested, motivated, and open-minded. The company pays you to learn their stuff.
So, that's simple : - First, rest right now ! - Second, when you'll be there, work hard !