Right, so you're stepping up a level or just starting out, either way if you're eventing you'll need to practise your dressage test. You've got to learn it first though....
When it comes to learning dressage tests trace them around a drawn out arena with your finger. If you're sitting at the bus stop and have forgotten an arena diagram there's an easy nemonic to remember the arena markers in a 20x40. Going clockwise from a - "All King Edwards Horses Can Manage Big Fences" sorted. If that fails then find a rectangular rug and run round it on a mini scale. You don't need to know everything by the letter as long as you know where you're going but it does help to know a few bits by the precise letter for reference. When you go down the centre line it won't look quite the same as when you practiced it so you need some reference points. Look for patterns and where double marks are as well.
When it comes to actually riding your test you should have identified the potential problem bits you need to practice. Do practice them but don't do them in the same place or the same way or else you run the risk of horses prempting you in the test - they're clever things those beasties!
Also, use the angles of the arena, there are certain areas of the arena that judges can't see as well. Particularly when your back is to the judge. These are the moments you need to use to make your adjustments. It surprising what you can hide in the briefest of moments.
When it comes to the actual test chances are there'll be a few surprises. For example, horses suddenly realise that they're at an event and think it's best if they call to all their friends. Even better, if you're on a younger one they might think the boards are actually jumps and they should be doing gridwork along the arenas.... Use these to your advantage! As long as you can keep them relaxed and not tense across the back you should be able to use their new found energy to get some better paces out of them with a lot more spring. Secondly, the most important thing to remember is to ride forward. Big bold riding will always score better than subdued nervous riding. At the end of the day it's about putting on a show, so make it interesting and be confident. You'd be surprised how far you can go on confidence.