it's nice that you will have time to watch before your classes start. That'll help a lot since it's your first time.
I do everything everyone else has mentioned in the days before the show...placing extra emphasis on getting all of my clothes clean, ironed, together, etc. same with tack. Ideally, I will have almost no loading to do the morning of (besides horse) other than just simply carrying clothes out to the trailer.
The morning of the show, since you aren't in classes til later in the day, you can probably just aim to get there right about when the show starts, maybe allowing a few minutes to unload and check in. Once you have done that, I would get your horse settled in where s/he'll be waiting and then you can go watch. Depending on exactly what you have left to do, estimate based on the typical lenghts of classes when you should stop watching and start getting ready. Remember, allow time for grooming, saddling, and warmup, as well as dressing into your show clothes and gettin yourself all neat and tidy. In my experience, it is best to be either finishing your warm up or waiting at the ingate at least one class in advance of your class, once again depending on how long the classes are running.
That's what I would advice based on my personal show experience; however, I have only been to local open shows, the all-around types, and I dont' know if other kinds of shows are a lot different or not.
Good luck, and I hope you have fun!
here is my heart and soul...please grind them into a hamburger and enjoy.