Please tell me where can i start to learn this language.
By doing the tutorials here. You can make the Roll a Ball tutorial first, then study all the scripting tutorials one after another.
After that: study all the other tutorials.
and
Then start unity. There is a prerequisite of adequate C# knowledge for scripting Unity.
I began learning C# while doing the Roll a Ball tutorial.
Well I have started to learn programming when I messed around in Basic on my C64 without substantial documentation, but it’s easier (and better on the long run) with proper introduction to the language. 
You don’t only know what to do, but you know why you’re doing that way and what other opportunities you have.
That’s very individual. Lots of people lean way better if they have an actual thing they’re working on while learning.
if you have no idea about C#. you can use some of visual scripting plugins like: blox,playmaker or so…
However, if you want to learn programming, best to avoid visual scripting.
Best to use visual scripting if you are already knowledgeable at programming…