first off GOOD LUCK AND HAVE FUN! I hope you get lots of pictures to show us!
I would bring for your horse:
All your grooming supplies and show sheen
A cooler to keep warm (if your in the US) when wiating for classes
Lots of clean rags to have someone wipe your horses nose and legs off and boots off after warm and right before going into the show ring
Hoof polish, you will want to put it on about 5-10 minutes beofre you start riding
Bring water buckets, a hay bag so your horse can have somthing to munch on during the break and waiting.
Extra clothes to wear over your show clothes till when you get on
Gloves and hair net!