Cowboy pants or Blue jeans & shirt.
Cowboy hat.
Cowboy boots- Optional.

Wear blue jeans or any western style pants. If you can find a cowhide pattern and make pants from this, it would be a nice touch. Wear a western shirt or flannel shirt, cowboy boots if you have them, a string tie and a western cowboy hat is a must. If available wear a neckerchief or bandanna and carry a lasso or rope. You can even put on a badge if you want to go as the sheriff.