I'm starting an office job on Monday, got to wear smart clothes - shirt and tie. Having spent the last few years in computer support I'm not used to wearing posh togs !
Could I get sacked for wearing polo shirt and jeans ? :-) (de rigueur in IT jobs !)
I would just put on a pair of trousers, shirt and tie for a few days, and see what happens. It could be that the dress code is not as strict as you think, and you can go back to smart-casual afterwards !
Beg, borrow, steal or buy the required clothing. You really don't wanna pee 'em off your first day do you? Then do as Mikey says, see what everyone else is wearing and follow suit (pun intended)