Showmanship and Conformation Line Classes
There are many different rules that apply to showing in hand. Specific breed or show rules will dictate the clothing you will be expected to wear. For smaller shows however, neat trousers or spotless dark colored jeans with a matching belt are acceptable, along with a crisp shirt, either in a solid or print fabric. I think conservative is better than flashy as you want the judge to notice your competent handling or your horse, not your loud print blouse. One color from top to bottom makes a long, clean, neat line that is flattering and not too flashy. Ladies, button up shirts further than you would for non-show situations. A neat cowboy hat in either a neutral or coordinating color looks nice. If you choose a straw hat make sure it is in good condition.
English riders in line classes can wear their helmet, shirt, jacket, breeches and boots.
Leather, rope or nylon halters may be allowed. Whether or not silver or other embellishments are allowed on the horse's halters will depend on the specific rules of the show. Those showing in English conformation or showmanship classes may be able to use a bridle rather than a halter. In English turn-out classes, the horse will be saddled and bridled.
Western Pleasure, Trail and Equitation or Horsemanship
There are some differences between what the horse and rider wear for each western discipline. But at your first small show, you just want to present a neat, clean appearance that doesn't distract from the performance of the horse or your riding skills. For Western Showmanship, conservative clothes with a monochromatic color scheme will look more 'quiet' than contrasting colors on top and bottom. Helmets are entirely acceptable in all western shows at the lower levels. Hopefully, this changes so that all riders may protect themselves at all levels.
A little more flash is allowed in Western Pleasure and this is one place fads and fashions change quickly. If you're svelte, a sleek, form fitting 'slinky' top in a color that compliments your horse would be acceptable. Blouses are the newest fad, along with some bling around the cuffs and collars. Darker colored pants and chaps are less distracting then light colored pants unless your legs are rock solid. The fringe fad is thankfully long gone, as fringes on blankets and shirts make the smoothest horse's strides look choppy.
Trail classes can be ridden in horsemanship type clothes, although some people may wish to add a bit more color.
Flashy is fun in speed games such as barrel racing, pole bending and key hole races and it's here riders can play with co-coordinating prints and colors. All clothing should be neat, with no puffy sleeves or loose fringes that could catch anywhere. Although some shows allow cowboy hats, more shows are encouraging the use of helmets. There are lots of fun helmet covers, or one can be easily made to coordinate your whole look. Shirts or tops should have long sleeves. Western boots or packers are acceptable. Some people may choose to wear chaps for extra grip. English riders who are having fun at gymkhana games can usually dispense with their jackets. Polo shirts may be acceptable. Because half-chaps have loops and zippers that can catch on things, they are sometimes discouraged.
A barrel racing saddle provides a deep seat so the rider can stay secure, but at the lower levels of showing any western saddle is acceptable. English riders can use their regular tack. I find it easier to do games from an all-purpose saddle than a dressage saddle, but you can use whatever you have, or is safe and comfortable.