First of all, start using it.
The easiest way to learn something, be it a computer program or a tool like a drill, is to start using it. Obviously you will need some safety instructions and basic terminology first, so start there: read some beginner tutorials and try to understand how things are interconnected.
When you fired up some useful containers, you may want to get into deeper details. You should start reading the official documentation on different topics like how process separation works, how network packets get into and out of the containers, how disks are used and so on.
Later, if you want to become the best of the best, you will have to read technical papers, then learn Go and analyse the source code.
It all depends what type of “pro” you want to be. But regardless of that, you have to start with the basics.