Write every minute you can spare, and keep perfecting it. Read as much as you can in the genre that you like to write in. This will expose you to great writing and perfect your own prose and punctuation. Note how another author develops character and advances the plot. Take the story apart and study the mechanics of it, then put it back together again and simply enjoy it.
And don't give up until you have your story written. Have another author or an avid reader critique it, and take their advice on how to strengthen the story or eliminate the excess. Polish, polish, polish; never be satisfied with your first draft, and don't be afraid to change a plot line or character's story if they feel "untrue" after you read the first draft. Keep writing until the story flows without any visible seams and your characters feel as real as the people you meet every day.