I'm one of the authors here who started with fan fiction, got comfortable enough to branch out into original fiction, and now have some work out there with a publisher.
Look around, find publishers who offer books in the genre in which you're writing (and don't settle for only the bigger houses), and read their submission guidelines. Submit, submit, submit, and when you do, take it down from any online sites. No one wants to publish a work that's available for free. Make sure you've read your manuscript, had a beta reader look it over, checked grammar, punctuation and spelling, formatted it properly, and GO FOR IT. You'll never know if you don't try.
And if someone does take your story, be prepared to toss your ego into a closet when you're assigned to an editor...