Specific shows may have rules about the dress and tack they require, but these are basic dress guidelines for lower levels English shows. If you're showing in Pony Club, breed shows or any higher-level competition, you need to peruse their rules to make sure your turn out complies. When it comes to smaller shows, however, my rule of thumb is to stick to conservative and traditional. That your clothes should look neat and well pressed is essential. I believe it's respectful to the judge and show organizers to take care with the turn out of horse and rider. Everything should be spotless and your boots well polished and clean.
Chances are you don't have to go out and buy all new clothing or tack. You may be able to find used items, or even borrow a few things until you decide what you need and want. One thing I haven't mentioned in the descriptions below are gloves. Unless you want to draw attention to your hands (and you probably don't) your gloves should match your sleeves as much as possible.
If you're an English rider but will be competing in line, conformation or turn-out classes, trail classes or speed games check out What to Wear to a Western Horse Show for tips.
The hunter world is one English discipline that strays a bit beyond the strictly traditional. While the conventional black velvet (ASTM approved) may be worn, jackets and shirts may be a little more creative. Something like a brown check jacket with peach shirt would be appropriate, although it's never wrong to go with hunter green, navy or black jacket with a white shirt. Coordinating breeches in tan, fawn, olive, brown or other neutral should be with a belt and field boots.
An all-purpose or close contact saddle is appropriate along with a regular English bridle. Depending on specific rules, certain types of bits, martingales or nose bands may or may not be used.
The dressage ring is very traditional, and each rider will be wearing almost identical clothing with little room for personal touches. Your hair should be neatly secured-you want to emphasize the smoothness of your horse's gaits making sure no part of your apparel, including your hair, bounces. A low bun or hair net is appropriate. If your hair is curly like mine, you'll need a fine mesh net to prevent straggling curls from escaping. Keep make-up to a minimum. Your helmet should be black velvet, or covered by a black velvet cover.
At schooling shows, you can wear subtly checked or modest tweed jackets, although deep navy is acceptable and black is more proper. Dressage jackets are rather longer than one sworn in the jumper or hunter ring. When you sit on a chair, the hem of your jacket should just rest on the seat when you sit nice and straight. Breeches may be fawn, pale beige or white. White breeches are the classic color for the dressage ring, but unless you have an entourage of grooms to look after your horse, it's almost impossible to stay clean. I'd rather show up in a pair of smudge-hiding beige breeches than dirt marked white ones.
Your shirt should be white and in very hot weather a tank top can hide beneath your jacket, while a snowy 'dickey' or bib and stock collar shows in front. A choker collar is also acceptable at low levels. Be prepared though if jackets are excused in hot weather. You'll want a neat and presentable shirt then. A stock pin is pinned so it secures the stock tie, and this is the one bit of personalization you can indulge in. A fine filigree initial or other subtle design is acceptable. I've also seen stock ties subtly patterned or in very light pastels at the lower levels.
Boots should be classic dress boots, not field boots with laces as worn in the hunter ring but if that's all you have, they are acceptable at the lowest levels. Kids can wear jodphur boots with gaiters. Unless the show is very casual, half chaps aren't appropriate. Of course, boots should be polished-learn how to spit polish, and make sure all other clothing is clean and pressed.
An English bridle and saddle are appropriate, and as you advance in levels, you will want a dressage saddle. Your saddle pad should be a square dressage pad, snowy white and spotlessly clean. Your horse's mane should be braided and bound with elastics or thread that matches its color. Leg wraps are generally not used.
The helmet and boots used for the jumper ring are somewhat similar to the hunter ring, except the jacket is somewhat shorter. Jackets can be in any dark color, subtle check or tweed. In some shows, polo shirts without a jacket may be acceptable. Field or dress boots may be worn with neutral colored breeches (beige, taupe, tan, olive, brown). Shirts may be worn with a tie, choker collar or stock collar and shirt tails should be tucked into the top of the breeches. Dress or field boots, or matching paddock boots and half-chaps can be worn. Children can wear jodphur boots with gaiters. Helmets can be any color and some may sport decorative embellishments, as long as they are ASTM approved. Your hair should be tucked up neatly in a net or bun. Pony tails may be allowed, but I feel that the fewer 'dangling ends' you have when your ride the better.
An all-purpose saddle may be sufficient for small shows, but a jumping saddle will help the rider maintain a secure seat.