Matching makes it look really good, so choose a color. This example's color is pink.
Pink cowboy hat
Pink bandanna cut in half around your neck
Purple/blue/whatever plaid shirt, tucked in
Light jeans tucked into boots (they are more casual than dark jeans)
Rope/jump robe hooked through belt loops as a lasso (pink ties the outfit together)
Wear any belt
Print off this belt buckle (or any other one you like) and use masking tape to attach it to the belt:
http://www.bucklecity.com/Western_Vintage_Star_p/bbca-5-p138.htm
You can color parts of the buckle pink to make it prettier.
If you don't have cowboy boots, just wear uggs
To make spurs, trace a dixie cup onto card stock (preferably pink). Cut it out, then cut little triangles into it like a spur. Poke a hole in the middle with a thumbtack. Wrap a piece of wire around your boot, then thread it though the spur. It looks really cool and usually spins! (sometimes it gets stuck)
Braid/pigtail your hair, and put ribbons at the ends of the braids
Hoop earrings look good
You should get tons of complements, it looks great!