If you are doing a schooling show, just make sure whatever you wear is clean, jacket is pressed and everything fits. A tidy appearance will go a lot farther with the judge them an unkempt but "color correct" appearance.
If you plan on doing more shows invest in a good quality jacket, something by Pytchley, Grand Prix, RJ Classics. You can try some new ones on at the store, note the size and colors you like, then check eBay
and consignment stores. It's better to buy one size up and tailor it down then buy a jacket that is too small.
A Navy jacket will always be in style. As will white shirts. Colored shirts go in and out of style, depending on the color. Essex Coolmax shirts are great for summer showing. Buy a long sleeve shirt because sweating on your jacket is just kinda really gross and not good for your jacket.
I wouldn't carry a Dressage whip. Make sure your crop is black.
Breeches should be "Tan" or "Green Beige". Each brand of breech fits differently so be sure to try on a lot and make sure to buy whatever flatters your form and feels comfortable to you. I wouldn't wear ribbed breeches and definitely not black. But as long as you are clean, and everything fits, if its a schooling show I've seen worse lol
If you wear a jacket they won't notice whether or not you are wearing a belt. A plain black or brown leather belt though is great to have and are inexpensive to get. I bought my first show belt at WalMart.
Like I said if you have time eBay, and always check consignment stores.