Welcome to the Sport of English Riding.  Are you showing Hunter rail classes or Hunter/Jumper?  If you are showing Hunter, you want to wear a Hunt Coat that accentuates you and the horse.  If you are riding a dark horse, then possibly wearing a light jacket will look nice, tweed color. If you are riding a lighter horse then go with the darker jackets (navy blue or Black)  Tan breeches are typical.  Tall field boots are the norm for adults. White shirt is typical, although I have seen the colored shirts shown as well.  If you are showing in schooling shows, not a rated show, you can show without a coat. Tan breeches, colored shirt, and half chaps are okay.  Check with the club you are showing, they typically post the riding attire requirements.  Show clothes are expensive.  Check Ebay, there are always good deals there.  If this is Dressage, everything changes.  You have to have a Dressage Coat, black, white breeches, black gloves, tall field boots or dress boots, and Hunt helmet with the bow on the back.  I forgot to answer regarding helmet for hunter, need to have Hunt helmet velvet cover.  Can be Troxel or similar with a velvet cover that is removeable.  I hope this helps.