What I should learn with dart ? and how long does it take to learn ?

We can't tell you. While we know where you are now (knows nothing about the subject, has not coded before) and we know where you want to be (can code well in the language) nobody (not even you!) has any idea how you think, and what aptitude you might have for coding.

And that's important - unless you have some aptitude you will never get there. For example, I cannot sing: I have no aptitude for carrying a tune, to the point where people run away with their fingers in their ears screaming when I try. Nothing I can do will fix that: and I have tired for several decades.

But coding isn't just about aptitude - it's a skill that you have to develop, a mindset you need to get your brain into. In many ways, it's like riding a bicycle: when you put your child on a bicycle, you know they are going to fall off, but that at some point they will develop the skill and be ba able to remove the trainer wheels. How long that takes depends on the child, and how much effort they put into it. Could be days, could be years - you just can't tell.

So will it be easy for you? No idea.
How long will it take you? Not a clue.

I do know that that just as you can't learn to ride a bicycle by watching the Tour de France, you can't learn to code in any language by looking at code others have produced - you have to put in the work yourself, and it can involve a considerable amount of effort because you are retraining your brain to think in a particular way and no-one can tell you how to do that! And the language you start with isn't that important; you can learn a new language pretty quickly once you have mastered one. Frameworks on the other hand take a long time to get proficient in as they tend to be huge!

So forget web and android, forget dart and kotlin: look around at what courses are available to you; what books you can buy or borrow; and start working.
I'd suggest that you start with C#: loads of good books on the subject, free and excellent development environment (Visual Studio), loads of targets such as web site, web apps, Android, Windows, as well as Linux - but that's a personal choice!