I'm assuming you're doing a hunter class?
Ride forward and 'present yourself' to the judge. I would then go for a short active walk, then trot and canter on each rein changing rein across the centre of your showing area and finally a (controlled!!!) gallop along the longest stretch of the arena if the grounds suitable (Judges or Steward will tell you if its not) - don't attempt any flying changes unless your horse is 100% going to get it right, judges don't expect it, just go back to trot and then into canter. Bring your horse to a nice square halt in front of the judge at the end and 'salute'
Don't forget to change your posting diagonal when you change rein
Keep it short and simple, judges get bored with someone who goes on and on especially in a big class https://www.youtube.com/watch?v=DV5FiVIDFbE https://www.youtube.com/watch?v=Tg4i3Fgldhc