Just do what your trainer has been telling you and have fun. When they ask for the next gait. Don't brake down to that gait till around 2 secs after they say you. You don't want you or your horse to look rushed. On the other hand you don't want to go to long. Try to stay on the rail if you can't. You look better and shows your horse better.
Stay away from other exhibitors, you don't want your horse to crowd up behind another horse (the horse may kick) and you want to stand out. If your bunched up with a bunch of other people the judge may not be able to see you well. When you first go into the ring pick a spot away from everyone else and if you feel like your creeping up on someone pass them with enough space between you or it's okay to make a circle to get away from the pack. I always feel awkward doing this but smile. It lets the judge know your happy and confident. Like Freemare said, don't be in too much of a rush with transitions. Make sure your horse is listening for your cues and try to make a smooth transition. And finally, be a good sport and have fun :)
HAVE FUN!! The ring you ride in is just another ring!!! Pretend like its the one where you ride in! Also it helps to bring a smile to your face if you think of the horse in front of you wearing something silly! Just smile and look confident! Don't go near the other horses stand out and remember everything your instructor has told you! The first show is always an experience. Just have fun and try your best!