I agree with the idea of just doing it, especially if you are trying to compete with yourself. People have different reasons for competing. I will say that most people that do a powerlifting competition can usually do double bodyweight squat and deadlift and 1.5x bodyweight bench press. Or if you want basic numbers 405+ squat and dead, and 275+ bench. Of course many, many things affect this like gear, bodyweight, etc. If you are younger like a teen those numbers will be lower.
Still, though, even if you are in last place I always say last place beats everyone that didn't compete and powerlifters generally cheer everybody on, from the best lifter to the newbie just getting his/her feet wet. If you are really nervous about competing go watch a meet and check it out. Good luck with it, it is very contagious once you get into it.