What breed is your horse? There's different criteria for English-type, gaited, or arab/saddlebred types. For a typical Western stock horse, you'll need a show outfit with a hat, shirt, belt, and showmanship pants. A leather halter, with or without silver, will need to be on the horse and not too loose. You'll need a leather lead with a chain, which you have to thread through the halter in a specific way.
Make sure your horse walks and trots by your side with a nice, clean halt. Teach your horse to stand square, and you'll be golden. The main thing judged is the horse's conformation, but most judges will dock points for an unsquare halt, breaking of gait, and incorrect presentation.