First off, congratulations on taking a risk and doing something new. Dressage is fun and engaging and I hope that you enjoy it. Remember that the "test" is a pattern that you ride inside the dressage arena.
The small arena is 20 x 40 meters which is roughly 66 feet wide and 132 feet ong. The large arena is 20 meters by 60 meters or roughtly 66 feet wide and 198 feet long.
The small arena lettters (used by Introductory and training Level dressage tests and Eventing tests) are All King Edwards Horses Can Make Big Fences.
You always enter at A. C is opposite A. Riding fom A to C is called going down the center line.
E and B are across from each other and notate the middle of arena. The imaginary letter X is located on the centerline directly between E and B. The link below has a really nice description and diagrams.
Most changes of gait take place at, near or between the letters.
Dressage shows usually require pre-registration and will provide you with ride times. You MUST present yourself to the the ring steward by your ride time or you will be eliminated from the class. If the class is running early and you are not quite ready you do not have to go early, but you can if they ask. When it is your time to go, you will enter the arena and go around the outside of the arena fence until the judge blows his/her whistle, honks his/her horn or whatever the signal is. Don't panic when you hear the signal..you have 45 seconds to enter the arena and start your test. 45 seconds is much longer than you think it is :-) You may have someone call your test for you during your ride with the exception of Championship classes and eventing dressage tests. You must remain quiet during your test and may not speak to your horse (no clucking). If you accidentally go off course the judge will blow the whistle and stand. Don't panic! The judge will give you directions as to where you went off course and tell you where to restart the test. This will be notated on your test, but you are not eliminated unless you go off course multiple times or if your horse leaves the arena with all 4 feet. Whew! At your final salute, the judge will stand. You may allow your horse to walk forward towards the judge (end of the arena) and then turn and walk out on a loose rein. Is is nice to say thank you to the judge on your way out.
About 20 minutes after the last ride of your class, (most of the time) you can go pick up your test. Each movement will be scored and the judge will have given you comments and written a sentence or two about your ride. Highest percentage in Dressage wins! Lowest number of penalty points in eventing wins.
More information on Rules for Dressage and some great basic information can be found on the US Equestrian website https://www.usef.org/forms-pubs/F3p8pgrWgAo/dr-dressage-division
How do you know what the judge is looking for at each level? At the top of the test there are a set of directives that tells you! Intro A is :
PURPOSE: To introduce the rider and/or the horse to the sport of dressage. To show understanding of riding the horse forward with a steady tempo and an elastic contact with independent steady hands and a correctly balanced seat. To show proper geometry of figures in the arena with correct bend(corners and circles).
All of the tests at each level are lettered in order ex. A, B, C - A being the least difficult and C being the most difficult. The most tests you would ride in a day would be 3. You can ride tests from two consecutive levels in the same day if you want to. Lots of people do this as they move up through the levels. They may ride Intro B, C and Training 1 or another combination that is better for their horse.
I hope this helps you feel more confident about your first test! Good luck and have fun!