D&D's beauty is in its freedom and flexibility so I'm happy for people to be creative. With that said, I don't have an entire world prepared! There's an adventure there for you and it's up to you if you want to follow it. If you don't then... well... there really isn't much else.
If people want to be dicks then they have the freedom to be dicks, but that doesn't mean freedom from consequence. If you want to piss all over the tavern, fine, but the burly barkeep is going to beat the trout out of you. If you go on a murder spree, the town guard are either going to kill you or put you in jail. Although who knows, maybe you could convince them not to.
Following what Fynn said, I've known everyone for years. I have every confidence that nobody is going to take things personally if they don't get things their own way all the time, or go out of their way to ruin someone else's day. I want you all to have fun! You dumb dumbs all mean a lot to me, in your own way. Maybe.
Perhaps to set the ground rules, PVP isn't going to be a thing so you can't attack each other, and stealing each other's best weapon is a no too. If you want to have some banter or play some pranks (I will be disappointed if nobody casts a spell to make it sound like Bubba is farting at an inopportune moment) that's perfectly okay though. It's up to you if you're comfortable with that beforehand.
Oh, and no smurfing weird sexual trout. Yes, you. You know who you are.