Congrats and good luck. Here's a basic list of things to bring:
Litter pan (can use a large box top and cut down to size, or a small aluminum pan - something you can throw away at the end of the show)
Cat's combs, nail clippers, cotton balls, Q-tips - any other basic grooming supplies
Small amount of cornstarch powder (for accidents or just to take static out of the coat)
Cat's food (2 bowls - one for food; one for water)
Bottled water or home water in a bottle - that way the cat will be used to its own water flavor.
Pens, pencil and or book/magazine to read inbetween times.
Small cooler with people food, etc. - show hall food can get expensive sometimes.
As for curtains, if you've never shown I recommend matching towels and black clips to hang them - you'll need enough towels to cover bottom of cage, top of cage and 3 sides of the cage. You might want to throw in an extra towel or to - never know when you might need it (accident, food/water spill)
If you are showing a kitten, you can make do with the normal single cage. For larger cats, I recommend paying for the extra 1/2 cage so you have a "double" cage - more room for you, a friend, and the cat. The club will give dimensions of the cage sizes.