nothing! if this is your 1st event with the car.
seriously, all sorts of people will tell you do this and that and basically make everyone rich in the process, and you the driver not really sure what your car is capable of never have driven it bone stock.
i've had two HPDE's in the STi this year and the only thing i plan is to install my COBB SS lines and hi-temp fluid. once the sotck pads are worn, maybe i'll upgrade to some Ferrodo's.
the STi is an incredibly capable car for the track off the lot.
my advice: drive it bone stock and then upgrade from there as necessary.
i would recommend you take a tire gauge with you. tire pressures for a road course need to be around 43 psi Front and 40 psi Rear once nice and hot.