Agree with Sky...have fun and enjoy the experience. You are hitting some of the high points...study your test if you don't have a reader. You should essentially be able to draw it in your head without the aid of the arena and letters.
Also, and this is a big one for me and always has been...BREATHE! :)
As a quick story I will pass on my first show with my last horse. He was an OTTB and still a little silly around things. The very first time I got him into a fully set up dressage arena, he did a great sidepass all the way down the F side..facing all the letters and snorting at them. In the week prior to the show I exposed him to everything I could think of..loud kids on the swings, loud music (houses border our ring), plastic flowers in the letter boxes (he had a definite aversion to any pink or purple..yellow and orange flowers didn't seem to bother him). So, show day comes around and we are good..he's calm, steady. Intro A went beautifully. Right before we go in for Intro B, people at the house closest to us decide to start throwing buckets of water off their balcony, and, another one started up a chainsaw. Well, we went down the centerline doing an impromptu freestyle...wiggle, wiggle, dance, passage (remember this is an Intro level horse at the moment)..snort, dance sideways..oh...X..OK. Stop, quick salute, sidepass to H (there is no sidepass in Intro :) ) Given that it was a schooling show I finally managed to get him somewhat close to the left side of the arena and completed the test just for the exposure, laughing all the way through the test at his antics. He was tense, up and just acting like a thoroughbred. Comments from the judge..horse was tense and tight...gee..ya think? :)