I will try to give you a quick run down which might be helpful.
When you weigh in you will turn in your opener for all of your lifts. Once that exercise starts you can’t change your opener so make sure it is something you are comfortable with and can do easily. I recommend people pick a goal third that is realistic and then go 87.5% for their first attempt and 95% for their second attempt. Make sure you know if the meet is in pounds or kilos. Once you try a weight you can never lift lower than that number so you can’t miss a lift and then go down, you can either stay the same or go up.
Once everybody weighs in they will post flights which are groups of 10-15 lifters that are lifting together. Most meets have 2-4 flights per competition lift but yours might be different. Each person in the flight is ranked by their opener from lightest to heaviest, being 15 you are likely to be near the beginning of the flight which is fine. Everyone will do their opener, then everyone will do their second, and then everyone will do their third. Most people, especially younger adults, need 15-30 minutes to warm-up for their lifts. The general idea is that it takes about 15 minutes for each flight to complete 1 round of attempts (or 45 minutes per flight) so most people start warming up when the flight before them finishes their first attempts. If you are in the very first flight then after they do a rules briefing (which you should definitely attend) they will say something like ‘we will start lifting in 30 minutes’ or whatever and then begin warming up right after that.
In general there is an hour or two between squats and bench and bench and deads, sometimes even more in bigger meets. There is usually a 10-15 minute breaks between each attempt, listen for your name to be called out so you are ready to go when it is your turn. They will tell you have 1 minute to lift once they tell you the “bar is loaded”, that might sound short but it is plenty of time as long as you are ready, if you aren’t in the room then it is a problem. Don’t forget to turn in your next lift after you complete the lift on the platform.
You are welcome to eat and drink whatever you want. I would avoid eating too much junk food, try to eat some good healthy food that you are used to eating. Drinking Gatorade or the like or having a protein shake is fine, you might bring a cooler with you. Most people eat something (after a good breakfast) after squats and/or bench press. Eat a little bit of food so you have some energy but don’t get stuffed. I would not worry about losing any weight to try to make a weight class at your stage, just go in and weigh what you weigh and have fun.
You need to bring a singlet and a t-shirt and shoes and socks. If you have a lifting belt and/or wrist wraps bring those. Some people like to warm-up in their shorts or sweats and then they take that off for the competition attempts. Your T-shirt must be tucked into your singlet and you likely can’t wear a hat when you lift or listen to an IPOD when you are actually lifting (although you can do that when you are warming up if you want).
Make sure you know the key rules and guidelines which are:
Once the judge says “Squat” you can’t move your feet or do anything but squat. Be a robot and simply listen to what the judges tell you for all lifts.
Squat LOW, hips below your knees. If a marble was placed on the top of your knee it should roll toward your hip, not toward your shin.
Once you finish the lift just stand there - show off what you did - don’t move or take a step until the head judge says “Rack”
Once you get a lift off or take the bar out of the racks, hold it until the judge says “Start” (not all feds have this rule). It is okay to have help with the lift off.
Bring the bar down to your chest and pause it there, wait for the judge to say “Press”
Push it up and hold the bar there, don’t re-rack it until the judge says “Rack”
Keep your butt down and your feet motionless (and likely flat on the floor)
For the deadlift there is no start command, just walk up to the bar once it is ready and pull it up.
Hold the bar in the locked out position for a few seconds, wait for the judge to say “Down” before you set it down.
Stand straight up with your legs straight, don’t hitch it, and lower it with some control once you get the down command. Make sure you use some chalk on the deadlift and don’t let any baby powder get on your hands.
It is probably smart to read over your specific fed’s rulebook for any little rules that might need to be followed.
You will have three judges watching you and after the lift they will signal somehow, usually with lights, if the lift was good or not. A white light means they thought it was good, a red light means they thought it wasn’t good. If you get 2 or 3 white lights, the lift counts. If you get 2 or 3 red lights, the lift doesn’t count. If you got a red light but you don’t know why, POLITELY go to a judge when they have a moment and ask them if they could explain what you did wrong. You might be pissed/surprised but control your temper otherwise they will just judge you more harshly the next time.
To score it they will sum up your best lift in each of the three events and that is your Total. If you get a 0 in anything technically you have ‘bombed out’ and none of the lifts you made previous to that count. Obviously the goal is to avoid this.
If you can find/seek out someone that is experienced that will help you that is great, or if you can find someone to come with you to the meet it can make life easier, another lifter is ideal. In addition if you see someone that seems experienced in your flight you might want to watch/mimic them - when they are warming up you should be thinking about doing the same thing, if they are relaxed then you can probably be too (just make sure the person is actually in your flight).
Lifters at a meet might appear ferocious and intimidating but everybody there, even the good ones, are a little nervous and they are generally happy to be at least briefly helpful to somebody new starting out as along as you don’t annoy them with constant questions or get in their space. Doing a quick introduction of yourself and then watching from a polite distance is probably the best way to go.
Powerlifting is really awesome and I hope you have a good experience. You started 4 years before I did so I think that will pay off for you. Good luck with it, if you have any other specific questions let me know.