What should we learn to become a skilled product owner?

Sometimes just starting to do something is more important than only thinking about it. Even if your pick won't be perfect, as long as you allow yourself to inspect and adapt those decisions along the journey, you will be fine. It's good that you already had something that might be a vision for you, that is more than enough to start. So pick one of the mentioned topics, or a book dedicated to PO, and start digging things, soon you will be on track. Best of luck! :)

My son is 5 years old now. My twins are 1,5. Before becoming a father, after the news that my wife was pregnant, I started to read all these books. Dig in deep. Find all the knowledge I could obtain. I felt prepared. I knew it all; I'll be doing this in this situation, that in that situation.

The first thing I learned after my beautiful son was born, is that it doesn't matter how much you have learned. The actual experience will be different than it was in all those books, all those studies,all those youtube clips. Sure, the base knowledge is really helpful, but in my opinion, there is NOTHING more helpful than actual hands-on experience. Start to do it, read books in the meanwhile and apply what might be useful. See what happens. If it's now useful, inspect why not and adapt to create a better outcome. You're dealing with such a complex situation, with people, with interactions, different backgrounds, nationalities, experience levels, politics and so on, there is no specific book or study that will fully prepare you.

I wonder from where to start learning thngs?

To be a Product Owner there must be products to own. Most organization's don't have them, and run projects with agile go-faster stripes sprayed on. Where's the empirical control of value, the validated learning? That's where to begin.