The Elementary OS is really powerful and beauty operation system, based on Ubuntu, and I want build programs for the OS. How can I start it? Any books, courses, step-by-step instructions etc? What do you can give advice to me for starting? I know the OS use Vala programming language, but I can't find any books for the one.
upd: Now I know on middle level CSS, HTML and PHP.
Thx for any answer, best
Vala is some kind of modern language frontend for C programming, primarily for the GObject world (but not exclusively). That means for programing in the GNOME ecosystem.
Not sure what you want to do - develop command line applications or graphical ones. For graphical ones you will want to learn the Gtk+ framework. It is based upon the GLib framework, which you would use if you want to create command line applications.
I would recommend start reading on the Gtk+ documentation:
https://developer.gnome.org/gtk3/3.22/
This is all in the C world because the framework itself in C. So when using Vala things will be a bit different (but not too much). It can be challenging doing both at the same time - but it should be a good read and excercise to get a basic understanding on how things work.