1) Search for "what is *language-I'm-interested-in* useful for?" on ddg;
2) Google the same thing 'cause you never know;
3) If it looks cool/useful and adds something to the tech I already know, I find a tutorial and follow it.
4) Trial and error on a new project that I will end up doing in another language because by that time I will find the new project so cool that I have to finish it in a language I use proficiently.

Every damn time.