First off, do you show english, western or both? I'll show you a sample list of what I bring to a 2 or more day show.
I like to bring feed pre measured in brown paper bags and put them in a plastic container so they don't get wet. Mark each one with what day it is and if it's AM or PM feed. It will help you keep track of how many you have. Also bring an extra just in case. Bring at least 2 bucket, one for feed and one or two for water. I also prefer to bring a hay bag if I don't know what the ground is like. Bring enough hay for one day longer than you will be there too.
My current English list includes helmet, hairnet, shirt (and collar), jacket, belt, breeches, socks, and boots (with spurs) for me. For my tack I make sure to have my bridle and bit, saddle, leathers and stirrups, saddle pad, and girth. It may be a good idea to bring an extra set of reins too.
For western, I include my hat, jacket, jeans/pants, chaps, boots, and spurs. My tack includes saddle, saddle pads, girth, and bridle and bit.
Showmanship halter, fake tail, and outfit for me and are always included.
Bring what you would use for regular grooming plus show sheen and hoof polish. Bring extra band and yarn if you plan on banding and/or braiding.
Lastly, don't forget to have fun. Try not to stress out too much and good luck!
---Elle, Zing, and Diesel--- my love, my life, my everything