Calm and patience I would get there a lot earlier to hand walker her around and show theres nothing to be scared of
Biggest tip? RELAX. You will find yourself stressed and your mare will get stressed because of it. Remember to relax
Hello Alyssa !
How old is your mare ?
Is she used to noise, music, people clapping ? Is she used to colorful fences ? Is she used to travel ? A first show can be quite stressful for a horse, so my tips would be to get her used to all these elements, because on the day of the show, you will both be a bit stressed out so it will be difficult to manage her if she is not already used to these elements. Maybe you could try to ride with music, try to recreate the atmosphere of a show at your barn, with people clapping etc, to see how she reacts. If the arena where you will do the show isn't too far, maybe you could try to go there and ride for a while in order to have her knowing the place when you go there for the show.
These are just some tips. Remember that anyway, in general a first show isn't your best show, so keep cool, be ready to any possibility and try to enjoy the day.