Write and polish your first story. Make it as good as you possibly can, whether you're going to try to get paid for it or not. Once your stories are published, you may not be able to correct errors or make other changes, and those misspellings will haunt you forever.
Determine the best market for your story, and examine your publishing options in that market. If you want to be published, you also want to be read. Make sure you are clear in your own mind what genre your story is and whether you should market it as adult, young adult, or children's fiction. Also, be very careful to note any erotic scenes in your story; some online fiction portals discourage or disallow erotic fiction because they have underage readers.
Sign up at the portal you're submitting to (Booksie, FictionPress and Writing are three to start with) and become an active reader and reviewer in both the story archives and the forum. Make sure others know you are writing a story and plan to post it, but don't be obnoxious about it; focus instead on helping or supporting other writers and building friendships.
Read your chosen portal's submission guidelines carefully. Some may require you to carefully mark erotica, while others don't care. Often you are requested to format your work in a specific way, or use a particular word processor or file format. Be sure to adhere to these guidelines. While your story may post fine if you do it your own way, stories that are hard to read as a result will anger your potential fan base at the site.
Once your story is posted, announce it to your forum friends so they can go read it. You can also publicize it on Facebook and other social-networking sites.
Read comments by other writers and readers once your story has been posted. If you think you need more help with writing, consider joining an online writers group such as the Internet Writers Workshop or, for speculative fiction writers, Critters Workshop.