You can go to medical school, train, study, be an exemplary student, but your first day on the job you will be the worst doctor in the world. There is no substitute for doing, whatever your vocation. It's painful and difficult (and people may die) but it's the only way.
Obviously you want to be as prepared as possible but to be honest knowing how to prepare only comes after you've done it (whatever it might be) and done it badly. People who have been through it before you may offer advice, but what's right for them isn't necessarily right for you. Because part of doing it is failing. You're going to fail, you’re going to fuck up, and it's not going to go well. Which is a good thing. It is also a very unpleasant thing. But it's also definitely for the good.
Like a child who's been protected from germs his whole life, a simple infection can end up killing him. You have to expose yourself to what's out there and the sooner the better. Although it's not going to be enjoyable, you have to jump in the water and feel like you're drowning before you can learn to swim. It doesn't matter how much people tell you that it's important or useful or fun, it won't make that feeling of dread and anxiety go away, but you have to believe you will survive (which you will) and do it.
With regards to writing, ignore that voice telling you that you aren't ready. The voice is right, you aren't ready, but readiness isn't a requisite for the job, and you will never be ready if you don't start and fail and learn. We all need to be encouraged and helped and guided, but mostly we need to practice over and over. And every time you’re told what you've just written isn't good enough, every time you get a form rejection or no rejection at all, you have to keep going and going and going. You will improve, you will figure out what you're doing and you'll get good. It will take time. A long time. And if you feel exhausted and lost and convinced you have nothing of any importance to say that any reasonable person would want to hear, congratulations, you're on the right path.
Nobody ever got good at what they wanted to do by waiting. Well, apart from waiters.