A few questions ...
Is this an NASMA show? If so, what's the rating?
If it's not a NASMA show, I think you'll be fine with just the pony tail and regular belt. You won't even need the necklace. For NASMA C rated shows, Probably the same, minus the pony tail - anything NASMA and you'll need a bun.
Now, for the NASMA A rated, we've got a totally different story.
Braided bun, with bobby pins and hairspray. As for the belt, you will need something that matches your boots, which won't clash with your outfit. (I personally hate brown boots and white showmanship clothes.) You will need a belt buckle IF your shirt is to be tucked in. If you have a formal Showmanship shirt/outfit, then you want to leave your shirt untucked, and no belt buckle. You don't even need a belt. If you tuck it in, you'll need a belt and buckle. Nice and clean, too.
As for jewelry, I went to the NASMA National Championship with no jewelry at all and did well. I don't see the need for it :)
Please let me know! I loved showing NASMA Donkeys and Mules, and would love to hear about others :)