Hair is typically in a bun or ponytail. There are lots of bun-making tools available if you struggle to get her hair in a bun on your own

I use this one with a hairnet: Whirl-A-Style BunMaker

There's also doughnut bun makers

You can even make a tool out of a sock!

depending on the length and thickness of her hair you may need a hairnet and some bobby pins too.

Also don't forget a water bottle and money (Often parents and spectators need to pay admission fees, and depending on how cautious the comp is being about COVID there may be shop booths selling fun stuff like dresses, guards, photographs, etc.)