Have you practiced showing?
Have you watched a show before?
Halter Class make sure you look confident, know what you're doing and you are presenting you're horse with style. Make sure you are always! Looking at the judge.
For western pleasure make sure you have those low speed gaits downpat. Make sure you're relaxed and always in the right gait.
For all classes keep you're head up and looking where you're going, show that you have pride in the horse you're showing and SMILE!! Make sure everything fits proper and you're using the right bit for the age of the horse. Clean you're saddle and maybe bath the horse if that's possible.
Just remember as a first time shower don't get too competitive. Set a goal and go for it, don't make those goals too extreme, you want to come out of the class with a true smile. You want to be proud of you and the horse and not fustrated.
Good luck I hope you have fun!!!