You for sure need chaps and a shaped hat. Please, do not
come to a big breed show with a badly shaped (or not shaped at all) hat. ESP if you are doing showmanship or horsemanship, a shaped hat is a must. No flat brims!
The norm for shirts at my breed shows (APHA) are shirts like these at the least: /tmp/
And ones like this at the most: http://www.showgirlsapparel.com/