I've never been to a comp and I don't know if I'm good enough to compete. My 3x3 Average is around a minute, My 2x2 average is around 8 seconds, and my 4x4 average is around 2-3 minutes, and I'm learning 5x5 and OH. I was wondering what you have to know how to do before walking into a competition?
The poster above mentioned cutoff times. Even if you can't make cutoff, you can still sign up for events. You just won't get to complete every attempt. They'll still stop you if you can't meet the time limit, so it's not like you need to worry about wasting people's time. And sometimes you get lucky and you make cutoff when you didn't expect to do so. Has happened to me.
You'll have a great time, so sign up and go. Sign up for every event you can actually solve and want to do.