Being a first show I assume it is a schooling show :) in that case, regular tack, grooming supplies, your horse’s feed, buckets, leather cleaner, etc... they are very lenient. Make a list before hand and check before you leave! It is so easy to forget something. If you want to go a bit further bring and a shaped show pad (white) and should wear light tan breeches and a neutral color show coat with cleaned boots and black gloves. The jumper side is way more chill. But either way it is your first show so enjoy it and don’t stress!
Everything u ever use. For jumping
All your tack, brushes, riding clothes. Spare show shirt and breeches if you have them just in case. Boots and helmet. Braiding kit, show sheen, fake tail if you are using it. Hay, feed, supplements and human snacks. Water and feed buckets. Shovel and bedding fork. Safety pins for putting your number on. Folding chair, sunscreen and a camera.