My main advice is trust your horse. That's what I did and Sonny was great!! Arrive early so you can walk your horse around the arena to get him/her used to it. If there are anything hanging on the sides touch it, let it make noise, and just do some groundwork (on a line) in the arena.
Then get him all ready and let him graze (if the grass looks good...don't let him eat the icky grass).
Go into the warmup ring probably 4 classes before yours (that's what I did), and slowly warm him up. Do alot of walking circles, trotting, cantering (if you plan on cantering for the classes), and get him used to working there.
Then when you are ready to enter the show ring, enter without any nervousness. I'm not sure about your horse, but mine will pick up all of my emotions and play off of them.
So just relax, have a good time, and just have fun! I went to the show not to win, but just to have a good time, and get Sonny used to traveling