Stay calm and show them around the property with you on he ground show them you aren't scared and they don't need to be either
walk them around. they will get used to the surroundings. make sure not to give them too many treats or they will always be expecting one
Give him/her treats and calm them down--if needed most horses tend to get excited
Give them a lot of treats and tell them they are doing good
Try to keep them calm. Walk them around other horses and people and let him explore new things. Let him get used to the sounds of crowd. Let if sniff things to let him figure out what it is and if it is going to hurt them.
Don't worry so much about the competition as, just having a nice relaxing time and trying your best. Because no amount of ribbons can bring you as much joy as having an amazing time with your horse
I recently did it with my lease ponys owners other horse. i just took it really slow and walked around a lot before asking anything except asking her to keep her attention on me.
Many trainers go to smaller shows before larger ones. One thing recommended to me by Bernie once is to take the greenies to shows and not show them. Instead, acclimate them to the crowds, sounds and sensations they may experience when actually showing. This is also a great desensitizing excersize and often gets the horse off of the property and more trusting of you. If you don't have that luxury, consider 10 minute hand walks around the show to show your horse what is going on in between classes.