Usually, stories get posted here as posts in a thread. It's best to divide a longer story into several posts, e.g. one chapter per post, so that it stays readable. You can also post your cover art in that thread as an image if you have uploaded it somewhere.
Alternatively, it's also possible to just post a teaser and/or description of your story and a link to wherever it's uploaded. However, this will usually result in less or no feedback.