For a small show to get you started, I'd go for nice jeans (no bling, prefer to sit at natural waist- no lowriders!), a nice solid button down shirt, and hat, if you have one. Helmet will be acceptable as well at most shows if you don't have a hat. Boots should be clean and in good condition, and match your outfit well. A belt that matches will also be good, but I'm picky about belts... some silver bling is ok, but not a fancy fashion belt with studs or something on it.
You say that you're wearing black hat and chaps, but brown boots- I'd try to find some black boots if you can. I'd stick to a solid color, like the burgandy one you posted in your other thread, or like the ones Rod's has.
I have a nice show shirt, but I'm seriously considering going for a white Rod's one for super hot show days.
If you can find black jeans and don't mind buying a pair just for the show, I'd go for that, but regular ones will be fine if not.
The important thing to remember is that you should show that you respect the show and judge(s) enough to make a good turn out, even if it's not fancy. Showing up with a clean horse and matching tack (preferably leather) and a well thought-out outfit will be ok. Think of the simple outfits the men wear at even Congress or World's, women are sometimes wearing very similar things now, and still placing well.