It's like swimming... you have to just jump in the water and learn as you go.
- You will get taken advantage of.
- You will get clients that fail to pay you.
- You will come across clients that argue about pricing, some to great degrees.
- You will get projects that grow well beyond what was explained to you.
- You will have clients that want to design for you and treat you like a monkey behind a mouse.
- You will hit all the same issues everyone hits. And, in fact, you'll probably hit them more than once.
Don't get discouraged. Everyone deals with this stuff. It's not you or your work, it's just the nature of client-driven work. In my experience, freelancing is a 60-40 thing.... 60% client relations and 40% work. You can't learn client relations until you start dealing with clients. Reading experiences and stories of client issues doesn't really help you learn how to deal with clients. Although they can make you feel a bit less isolated, it really just takes practice and experience to handle clients well.
Start with small, safe, jobs - a flyer, a business card/stationary, a logo and you'll progress to larger jobs as you feel more confident. ALWAYS hit deadlines, and I mean always. Never yell at or insult a client, no matter how much they deserve it.
You're "ready" as soon as someone is willing to pay you and you understand all that's needed to complete their project. Don't take on projects at the beginning if you don't understand how something is going to be reproduced and you haven't agreed upon pricing and deliverables in writing.