Always open with a light weight in the squat, and always go convincingly below parallel. From there hit your gym max then a PR.
Bench, Open light. A few weeks leading up to the meet practice your opener with a pause, if your federation requires a pause on the chest. You'll be suprised how much a pause takes out of your bench. I was suprised.
Deadlift, You'll be pretty tired from a long day, nerves, max effort lifts, etc... so Open fairly light.
Your goals at this meet are to have fun, meet and learn from experienced powerlifters, and most importantly put up a total. You may have specific goals in the squat, bench and deadlift, that's great you should have those goals but those are secondary to simply posting a total. The last thing you want to do is bomb out on your first meet.
This article may help you pick your attempts and give you a good idea of how to warm up for your attempts. It's best to have a plan going into the meet. If you think you can just wing it you are probably wrong. Nerves will get the best of you. Have a plan and you will eliminate a lot of the first meet jitters.