This is a general rundown on what to do at an Australian dressage club day. This may or may not be an official competition, and at higher levels or non-club run events the protocols may be somewhat different.
When you arrive, you should park your float next to the other ones. Unless you're horrendously early, there should be one there: at least one person who runs the club will be riding. If you are unsure, park your float, leave the horses in it, and go check in at the club room. This is probably a shed, and generally has food cooking nearby. Follow your nose.
Find someone at a table with pens and paper, or someone with a microphone, or someone who doesn't look lost. Ask them where you check in to say you are present. Very often, riders don't do this. It means the judge might not know if you've scratched or not, and creates confusion. We don't like confusion. Sometimes you don't have to pay previously, so you should do this now. If you aren't sure if you've parked your car in the right place, ask them.
Although you should have found out previously, double check which ring you are in and what time you are on. Notice the judge's name. Look at at least two horses who come before you, and find out if you know them. If you know at least the horses' colors, it'll help you find out if the ring is running late or early.
Now, go get your horse out, walk them about a little, take off rugs, let them settle down. Then tack up, warm up, and go to your ring.
You should do much of your warming up close to the area you are going to be riding in. For example, if your ring is close to a line of trees, ride near that line of trees so that when you are in the ring, the horse doesn't freak out. But you should not ride around the ring towards the judge (who is generally sitting in a car) unless you are ready to present and perform. The moment you go down the long side of the ring you are saying to the judge, I'm here, I'm ready.
If you ride around the car, you're saying the same thing. If you ride around the car, and the judge is busy finalizing the last test, ride around the car or ring a couple more times, but don't disappear up the end of the ring to (seemingly) never return.
To present to the judge, go to the driver's side (which is probably where the judge is sitting) and say hello, give your name, your horse's name and ask if they want your number. If you think you are early or late, apologize ask if it's okay if you go now. The judge will turn to the penciller, who'll check their schedule and say yes, of course you may ride now.
Do your test confidently. If your horse is being a complete nutter, bow out gracefully. The judge would rather you dismount then be dismembered.
Once you've done your test, warm down, dismount, untack, tie the horse up with some water and hay, and go to the club room. Here, you should have lunch, and strike up a conversation with somebody. With horse people injury, weather and How did you go today? is a good place to start.