Two stories - one a cliche, and one that happened to me, which I think are vital.
First one - a man is new in town and walking down the street looking up at the buildings, feeling lost. He asks someone who looks like a local 'Excuse me sir. How do I get to Carnegie Hall?'
The stranger thinks for a moment, and answers, "Practice".
Ok, Ok, I know you've all heard that one. Here's one that happened when I was fifteen. I had gone through school believing that I had no artistic ability (due to teachers never encouraging me, low self esteem, etc). One day I was sitting with someone who was a fantastic visual artist - he already had a couple of comic books published at the age of about sixteen, and he was 'doodling' these amazing images. I commented to him how much I wished I could draw, and he said "the only way I go this far was to work on it every day."
I truly believe that everybody is born with imagination and the potential for talent, but it comes down to whether you want to commit yourself to developing a particular skill. Sure, some people will naturally be the master violinist or have the ear for brilliant poetry, but everybody has the chance to express themselves in a unique and wonderful way if they have the time, and TAKE the time, to train themselves.
The question of what makes a good STORY, as opposed to a good WRITER, is a whole other issue ...
EDIT: Oh, one more thing to make a great writer - READ, READ, READ!!!