I’m not the best person to ask right now because I’m not earning a living from it yet.
The only advice I can think to give you is: read a lot of whatever genre you intend to write (investigative journalism, fantasy novels, whatever), and make friends with someone who is already successful in your chosen field and can be a mentor of sorts.
There are also lots of articles online that can teach you how to do basic things like pitching magazine articles or writing a cover letter.