I always take a friend usually one who's already competed so they can help with advice and to help calm the nerves but always remember your doing it for fun I just focused on going clear and gaining confidence in my first few shows.
Organize yourself well! I recommend cleaning and prepping all your tack the night before the show. Walk the course twice if possible and visualize it a fee times over, planning your turns and distances. Breathe deep, even if you're nervous af, haha. Thats what i always used to forget and my horse felt it and got kind of panicky. Also: Eat enough! Don't underestimate that factor! You need strength and energy to exel. Best of luck:)