At shows you are given a ride time, you do NOT have to ride earlier than that time (which you could if someone scratches or classes run faster).
So look at your ride time and based on that back it up to determine what time you are getting on the horse (so you have plenty of warm up time), what time you'll start to prepare your horse, etc.
For example if your ride time is 8 am and it takes you a good 20 minutes to barely warm up, another 5 minutes to walk and relax, then another 15 minutes to get horse going well - you'll want to back up the time you get on the horse by 40 minutes (40 = 20 + 5 + 15) - so for an 8 am ride you should be mounting by 7:20 am.
Warm up somewhere quiet - spare ring,...somewhere you you can perform some of the same movements you'll be showing - open field for 20 meter circles, warm-up arena, etc.
Once the horse showing before you starts walking after their final halt you may enter the area roped off directly around the arena. That horse must exit the arena (and the area directly around it) before the judge will whistle for ring a bell indicating you should enter the ring (usually at a trot - but what gait you enter at is driven by the test you're riding).
You ALWAYS enter at "A", halt at the letter the test indicates (usually "X" which is the middle of the arena), SALUTE (both reins in 1 hand, other hand goes straight down riders leg (thigh) and rider looks down, then up, re-distribute reins to both hands (judge should have "bowed/nodded/waved" to you by then) then continue riding test.
At end of test halt where test indicates, salute once again, then exit arena on long rein.
Your first few times it's best to look where the letters are so you can make your "patterns" perfect - i.e. make your circles a circle not a square or rectangle, etc.