No one cares. Dress how you normally would.
You might get funny looks turning up to freshers fair in heels and a dress that barely covers your arse, as it's usually in the morning/afternoon and everyone is hung-over and has just rolled out of bed to see if they can scrounge any pizza.
Similarly if you go to a super-club with a bunch of girls who like to go clubbing dressed in baggy jeans and a stained hoodie you might get a few raised eyebrows as they'll be expecting someone to dress up.
In reality, no one really cares and it's the same as any other time at Uni. Going out = whatever you'd wear to go out. Wandering around campus = whatever you'd wear for wandering around campus (some girls might wear pajamas, other's might spend 2 hours getting ready every day for lectures)