How do I start writing down my idea for a story?

At PAX East 2014 I had an idea for a story I wanted to write, loosely inspired by DayZ adventures witnessed and imagined. Over the last year, I've tried but I just can't seem to get it out of my head and put into words. The idea is like a tantalizing mist; I can see it, but it's not substantial enough for me to grab hold of and do something with it.

Not being able to move past the most basic outline of this project frustrates me very much, and I don't know how to proceed from here. I guess I need to learn storytelling, how to structure and build a story and things like that.