Since it's a small, local show, I wouldn't go too over-board. But...Here's what I'd do...
Your horse will have to know how to walk, trot and stop while being led. Transitions should be precise and fluid.
Teach your mare how to "set-up" and stand patiently. Front feet should be together, back feet should be together. And you shouldn't set her up in an uncomfortable way. Her legs should be straight underneath her. (Like this)
In halter classes, you are able to touch your horse and physically pick up a leg and place it to where it needs to be. (If your having difficulties)
Groom your horse very thoroughly. I will do this the night before and then put a sheet on, standing wraps for the legs and keep them stalled over night. A sleezy is often used to keep the mane on the correct side and their face/neck clean. Many people band the mane but that might involved cutting your mare's mane and if you don't want to do that, condition it very good and comb it so it's nice and silky.
Use a nice halter and lead rope. Either black nylon, leather or a leather show halter with silver. Use a matching lead rope.
Here are some youtube videos to watch that will explain what a judge will do, and where you should be when a judge is walking around the horse. Always smile and be polite. Big cheesy smile too! Say "good morning" or "hello" when the judge comes over to you.
You can get an idea of what people are wearing in the videos too. Typical attire is boots, slacks/dark jeans, a blazer and cowboy hat. I always wore a bright colored top when I showed to grab the judges attention.