Walk her around the arena and stables to get her used to the new environment.
School her around other horses, try and ride with other people around in the arena or in busier places. Maybe get your friend to stand on the ground during your ride and make jumps, flick poles and do other busy things that would be happening at a show .
I would go to schooling shows first so that she can get used to the busy environment.
When u and ur horse are getting nervus just sing a song to keep the attention on what u r doing. Works with me
introduce her to things like flowers that may be there or really anything else "scary" and when you get to the show ground walk them around everywhere focusing on the ring you will show in
Take her to the show grounds before your show to let her see everything. At home set some different jumps up for her to get exposed to them. Also taking her on little cross country schooligs really help also!!!😊😊
Keep her focused, don't let her get distracted by other horses or it will be hard to ride her. I would suggest getting her used to the jumps and people and horses in the arena. If you can try and do some schooling in the arena before the show, just to get her focused.
I always think that it's best to rent a show centre first so you can slowly introduce them to the coloured jumps and different place with out the stress of the show and lots of other green horses, u can also start of as small and make the course as easy as you like . This way is much less stress for the horse and when they do go to their first show it's not as stressful for them