Which's the most important skill you should learn if you want to work for yourself? ...

Selling.

I don't think you learn this by reading about it, but by becoming determined to figure out what motivates people to take out their wallet and pay for things.

I started by observing my own behavior when I came to make purchasing decisions. Whenever I found myself choosing to pay for something (or choosing not to), I would try to reflect on why I was making that decision.

Even though everyone is different, we're much more similar than we think, and studying yourself is a good heuristic to start learning how to sell.

The next step is to have something to sell, and experiment with your marketing.

But don't just try random experiments and hope for the best. Try to do things that you feel reasonably convinced they will work (based on your observations about your own buying motivations). If they work, you learn something; if they don't, you learn something as well.

If you don't have something to sell yet, try to make people do something for free, such as clicking a link, reading a post, or joining a mailing list. This is still good practice for figuring out what motivates people to do something you want them to do.