Typical show clothing at local shows are dress pants, usually black for hiding dirt and make up, and a nice jacket and shirt. Most people don't wear a lot of "Bling", but it's acceptable. But it's nice if you can find a shirt or jacket with some rhinestones and spangles. Watch for glitter - you have to make absolutely sure it won't shed or that can get you disqualified. Just be careful that it's long enough so when you bend and reach to set feet, you're not showing more to the judge than you want to!
Hats are not usually worn by girls at halter. You will need leather shoes, half boots, or full boots, but cowboy boots aren't required.
After holiday sales or Good will or Salvation Army is a great place to find shirts and pants, and occasionally jackets. Then you're not out much when they get dirty or ruined.