What Language should I learn for for back end developtment?

I know most popular languages are c#, PHP and python.
When it comes to competitive programming I think python has more opportunities.I’m really stuck in chose.Please what should you recommend as my mentor.Please also provide guide lines that I should follow?how can i grow?

thanks in Advance

It depends on what you mean by “back end development”. What do you want to build?

Sir,I m basically looking for entry point for back end development of web applications.There are lot of languages ,I cannot decide.
Sir what would you chose if you would have to select programming language as a beginner?

I did chose Go (Golang). Almost as fast as C and almost as simple as Python…