You don't know you are ready (ever), you trust your training, you trust your bike handling, and you get in there and race. If you don't get dropped, you were ready! Eventually, you have to work on tactics (positioning, when to attack, how to time a sprint, etc) to really put together a win, but that comes.
I recommend joining a team with experienced racers as they can help you with training, tactics, skills, etc. Don't enter a crit until you really know how to corner at speed in a pack!
Training for racing involves a lot of speed work as races involve a lot of accelerations. If you haven't done this type of training before, it can really help to work with people who have.
The road race season is over in most places, but it is actually a pretty good time to join a team. Train through the winter with your new teammates, get to know everyone, and start racing together in the spring.