I also think that people who are already parents...tell you (endlessly, smugly and at great length) that 'nothing prepares you for parenthood' and that 'you can never imagine how much your life will change'. How can be absolutely certain that I want something that I can't even imagine?!
But it's true. You can't prepare. You don't know. In fact some of the parents who find it the hardest are the ones who felt they were the most ready! I do wonder in some ways if people who go into it with ambivalent feelings or a serious sense of apprehension actually end up tending to enjoy it more because they were expecting it to be hard and found it hard to anticipate the nice parts aside from a vague feeling, so the hard parts mostly meet their expectations with a few spikes where they're worse and a few lulls where it's easier, and the nice parts blow them away because they're so much better than they expected.
Yet someone who yearns for and has always wanted children gets a bit of a raw deal because while they tell themselves they know it's going to be hard, they are often romanticising it, assuming they'll ace it and basing everything on the wonderful time they are sure they will have and I think the reality is somewhat different.
Being ready I think is a misnomer because you can't be prepared but there are things which can help:
Having a genuinely stable, supportive relationship with a good partner OR having good support around you if you're going it alone. Don't do it if you have a relationship which is in any way in question. Just DON'T. It's not worth it. Get a sperm donor and do it alone if you have to, but don't rely on someone who's unreliable.
Having a bit of financial security - you don't need to be loaded but babies throttle your earning capacity and they cost you money so it's useful if you're prepared for that, just makes the whole experience less stressful I imagine.
Being aware that it's going to be a challenge and I don't mean nappies and sleepless nights, I mean that it's basically a constant puzzle that you have to keep figuring out and it keeps changing all the time and it never really ends. I actually like that about it but if you think that parenting is something you just have to figure out and you can get it right and then sit back and relax, no.
Being aware that a lot of it is boring. Children aren't as constantly entertaining when you have them all the time. Practically, you're adding a lot of mundane shit to your life because they don't do their own housework, cooking, or even personal hygiene for a long time.
Accepting that you're not going to be good at it. I don't mean that you're going to be an awful parent, it's just that so many things in life it's easy to excel at and parenting it isn't. Partly because it changes all the time so as soon as you get good at something the requirements have changed, partly because there is no definitive answer to how to be a good parent, partly because it's just not the kind of thing you can "get an A+ in". It's a relationship, not a performance.
And then yes having reasonable expectations in terms of free time, career development, relationship time, travel opportunities etc. None of this is dead forever but being okay with having it all curtailed is important.
I think if you're okay with all of the above then you're probably as ready as you can be.
I also agree with a PP who pointed out that having one child is very different to having children. It's a different lifestyle and much easier to retain some of the adult orientated stuff.