I posted in the other thread. If you're planning on getting new wheels, I would maximize the allowance from the STX rules and go square 18x9 wheels with 265/35 tires. However, if you want to stick with your OEM wheels, then go for 235/40 front, 255/35 rear. Yokohama Neova AD08 would be my first choice, Dunlop Direzza Z1 Star Spec would be the second.
And since you mentioned that it was your first event, I would recommend you to get more seat time after you sort out your tire selection.