Try to distract yourself with music,looking at the course, or even brushing your horse
Make sure you get everything ready and packed the day, or a few days, before the show. That way you aren't rushing in the morning. Also make sure to get a really good nights sleep! Good luck!
go over your course a lot! you'll be fine!
First make sure you had enough practise before the competition. Then the day before you shouldn't jump or practise what you will be competing. Maybe do a light flatwork or go outside to relax you and your horse.
Then make sure all your stuff is ready, you and your horse equipment clean and ready to use.
Have breakfast the morning of the show and be careful to know the hours of your round so you won't be late.
Then the last thing you have to do is to enjoy your day, watch other riders and have a good time!
Good luck :)