You’re ready when you are able to stand up in front of a crowd and show your stuff. you are usually free to enter the free styles or the sports ladder.
^
what jhb said.
Anyone can enter most ladders or freestyle contests. Just note that standing up on stage in front of (depending on what contest you go to) dozens or even hundreds of people, can be very nerve wracking. Especially the first time. Just try to relax, and have fun with it. Practice your routine until your can basically do it in your sleep, should help a bit even if you are nervous.
you should start out with a nice big number(not too big that you take 1001 yoyos) but I think 5 for the first time would be ok. Don’t get too nervous about getting up there, even the Great Ben Conde get super nervous(the biggest factor that contributes to his mess-ups)
it depends on the contest. A lot you just go to the website, and there should be a section for entry. Or some have it where you can sign up when you get there.