How can I get paid for a PhD in Europe? I'm confused: someone said that a PhD in Europe is free, someone said that you get paid. What's true? And how hard is to get a PhD salary in Physics in North Europe?
You go to their website, find a job posting, and apply. Example of a PhD Research Fellowship at the University of Oslo.

Not all positions are free, and not every PhD student is paid. You'll have to look.