Not knowing your skill level it is hard to offer advise so I will offer it to you are a true beginner. A lot of shows have walk trot warm up classes. I would start there. DO NOT go into the rail classes if you don't have good control at all three gaits. Stay with in your skill level and you will have a fun day. Go beyond it and you might wonder if you ever want to do this again.
Halter class is a good way to start the day. Judged on your horse's conformation, there is no pattern. Watch the class ahead of you to see how the judge runs his line up or ask someone.
If you feel the Walttrot class is a good fit and the Walk-jog-lope is a bit more than you want to do, stay and watch. Watch the class. Pick your winners and see how they stack up to the judge. You might not agree but ususally the better mounts rise to the top.
What to wear? If this is your first show and you are lacking in show clothes(I make custome designed clothes and have dressed world champs so this I know) keep it simple. A judges especially a male judge appreciates clean lines and simplicity over trying to make thinks go together that do not match. A pair of creased wranglers(yes iron or better yet starched) with a pressed or starched shirt preferable in a plain color-can never co wrong with white or blue and men's oxfords at Walmart will work- with sleeves long enough and can be tuck in well, a belt, clean boots and a decent hat. Can be straw or felt but NO WAVES to the brim. If you want a bit of color or to soften your look, a pretty scarf tied around your neck. No floppy ends. It should be wrapped twice around and tied with a small knot at the front. Pretty stud earrings-no dropped ones. Hair needs to be contained well either is a nice ponytail or a tidy bun with no flyaways. Just like showing cattle, that first impression of cleaness and tidiness go a long way.
Clean equipment is a must as well as a clean clipped horse. Ears, bridlepath, muzzle, fetlocks. A shortened mane is preferred but if you have a mane you cannot part with, it sould be laid down nicely.
Most inportantly set a small goal for yourself. Even if it is " I will go into my class with a smile at the start and finish" and talk to people and have FUN!! Joan