Howdy Rupert,
I get this asked a lot as photography's the world's biggest hobby.
Best place to start is with what sort of photographs you like taking? Portraits? objects? landscape? motorsport? wedding? fashion? journalism?

Main thing to know is that being a professional photographer is as much about business (advertising, money management, etc) as it is photography.
Get a bit of a portfolio, online and hard copy. Websites are a good shop window. Build and print a few flyers and then pop them out with the papers. See what happens. Good luck with it all pal.