I would suggest finding a trainer to take some lessons from. You need to have proper form over your fences and equitation on the flat. Even if it's just a few lessons, it will help.
As for attire, I would go with light colored breeches, a white long sleeved english shirt, and either tall boots or black paddock boots with black leather (smooth leather) half chaps. You shouldn't need a jacket for 4-H if they're smaller shows, and you should be able to get away with paddock boots and half chaps. If you want tall boots, get PVC or rubber boots to save money, and buy a nice used jacket on eBay
. And of course, don't forget your helmet! If you can't afford a nice velvet show helmet, then get a schooling helmet and buy a velvet cover for it. That's what my daughter shows in and she does fine