The jumps will be conservative and "natural looking". There will probably be flower boxes and maybe some brush piled at the bottom of the jump. No bright colors or crazy 3-D shapes or anything.
You should wear a show shirt and jacket, colors that coordinate nicely. Beige breeches, preferably side zip, with a belt. Shirt tucked in, black gloves. Tuck your hair in your helmet, use a hairnet. DO NOT WEAR A SHOW BOW! Shine your boots, clean your tack. Braiding is optional at schooling shows but if you're good at it and you want to give it a shot, go for it. You should use a white fitted saddle pad, not a square pad and not the kind where the number slides in a slot. The horse cannot wear boots of any type. He can wear a standing martingale for O/F classes but not flat classes. He should wear a plain snaffle bridle, no flash or drop noseband and no crazy bits. A D ring is the most "hunter-y" type bit but any simple snaffle will do.
This video is a good example of a schooling level hunter show. The long stirrup refers to the age division of the rider. You'll notice the rider attempts a flying change but the horse cross canters instead, she fixes it in the next stride or two. Simple changes are probably acceptable at a schooling show but flying changes look much much better.