There will be a ring steward to tell you when and where you can do your trot around, and also they'll usually specify whether it's a whistle or bell or horn to signal your ring. Sometimes you'll be instructed to ride your warm up lap inside the ring, sometimes outside, it just depends. Usually you can enter or start to trot around after the rider before you had saluted and started to walk on a loose rein. Don't be scared to ask if you're not sure, you should also plan to be warmed up and arrive at your ring a few riders before you, so you have a few minutes to acclimate your horse, have a trot around, and watch the rider or two before you to see how things go.
Good luck!! I hope you have a blast