# How you become an expert in any specific programming language from a 
# stage where you don't even know how to write "hello world" in that 
# language?

Given how the question is worded, I will suppose you already know how 
to program in another language.

First, write that "Hello World". Good, you have made some progress.

Then find a good comprehensive tutorial-style webpage or book and 
follow it.

Then find a toy project in that language and work on it. I advise 
targeting a few thousand lines of code. It can be anything you like. 
Suggestions: a LISP interpreter, a video game, a client for a network 
protocol...

Then find reference projects in that language and read their source 
code. Say about five 5000 to 50k LOC code bases, old enough (at least a 
year), with reasonable usage and if possible with several contributors. 
Observe the patterns and code style.

Then find a real project to work on. It can be professional or it can 
be a side project, but it has to be serious and you have to put the 
effort and collaborate with others.

After that you will not need that answer anymore, but I would say: 
write Open Source code, go to conferences, speak, blog, become part of 
the community of the language.

And finally, read the source code of at least one implementation of the 
language, to the point where you can understand exactly what happens 
when you run one of your programs. I would not consider anyone who has 
not done that an expert. And yes, it means it is easier to be an expert 
at some languages than others! :)

    What I cannot create, I do not understand.
    -- Richard Feynman