Wear a suit. As it is your first day, it is highly likely that you will be taken around and introduced to people. Do you really want to look like shit?
You might go a little dressier than the normal code on the first day, but if there’s an understood dress code or expectations, it’s best to try to fit in and not appear overly out-of-place.
Don’t do a suit if you already know the standard is jeans and a T-shirt without coffee stains. But you might go in with a sportcoat and button down if the standard is khakis and a polo shirt. Or add a tie if they already do sportscoats and button downs.
You did have time to observe what people on your team were wearing when you interviewed, yes? Or did you not interview on site?