+1 on info provided by Dr J and ouroboro.
When you find a federation you'd like to compete in, make sure you read the rulebook. Most federations have extensive and detailed rules. Start to practice your lifts competition style, like paused Bench and Squats parallel or slightly deeper. In a paused Bench, stay tight. If the bar keeps sinking down slightly, they won't tell you to "Press!" until it's stationary. If you are not sure that you are squatting deep enough, or pausing your bench appropriately, have someone (with powerlifting competition experience) watch you or call the lifts.
On competition day, don't exhaust yourself with too many heavy warmups. Warm up sufficiently with a low weight, you just need your muscles, joints, and tendons warm and fresh. For your first lift, stay with something you can walk into the gym and hit any day. Then your second should be something you can normally do. Then your last can be a personal record if your feeling up to it, or slightly lower than a PR if you want. The goal is to complete all three lifts, however don't be discouraged if you miss your last lift. Feeling nervous can be your friend, or it may mess with your head the first time. You won't know until the day comes. Most people feel even stronger on competition days, something about the energy in the air.