Click on Stories -> My Stories in the top menu bar to see a list of your stories.
Click the title of the story you want to delete. Now click on Delete story in the right-hand menu.
We also want to point out the possibility of simply hiding your story. You do that on the same page you can delete it. This will make your story invisible to other people.
Note: If your story is currently in review, you will need to use the Place in Older option before you can delete or hide it. (The Place in Older button takes the place of the Delete Story button for current stories.)