Start off with small competitions and gradually build it up to bigger and busier ones
Lunge them and walk them around the grounds
Good luck! But I always use a older horse as leader for the first times, I do the same with the firsts competitions, to show them that all is ok
Bring an experienced and calm horse with you! And make sure your horse is in a relaxed mode when you go out! And try to find a day with no wind and rain
I would say be prepared for anything. Bring a lead rope with a chain so you can put it over the nose if they get exited. Also, just walk them around get them used to there surroundings. If your going to a show I would also say lunge your horse.