Their's a million ways to write a song. Me, I ususally start playing something these days, not a fully formed idea, but atleast 50% done. But I can't say "I'm going to write a song now" and sit down and do it, rather songs just pop out of me when I'm in the mood.
It all started with trying how two chords go together, then adding a third and so on until I had a progression I liked. Just mess around with stuff and eventually you'll come across something. As time goes on this will come more naturally.