Belive in yourself and your horse, he knows what he's doing  

I would just do an introductory test (just walk and trot) and/or a preliminary (walk, trot and canter) test (if its BD), if possible, take the horse you are going to compete down to where the competition is being held before hand so you can work your horse in the arena so both you and the horse are used to it. Just remember to smile and breathe during the test, and make sure that you've warmed up properly before hand so both you and your horse should be relaxed. I would also make a list of things to do/take so you dont have to worry about forgetting anything (you can ask some one to check it incase you think you have missed anything off the list, but do it before the competition, there are online checklists but I find that they have alot more things on them than I need). Make sure that you know the test inside out as well (I find writing the test down, drawing it, or pacing it out in the living room helps as you cant go down and ride it all the time, as your horse will learn the test too - which isnt a good thing as they anticipate things and tend to do them too early. Finally, try and go with some people you know (with horses from the same stables as yours because of strangles) as you can all help each other out.