Not used fanfic.net before? There's really nothing to it! You have to register first and foremost. Give yourself a penname (author alias) and set up your personal setting (i.e. password and all that jazz). Then, you go to the "Document" tab and from there, use the uplink facility at the bottom end of the page to upload your work from your computer (e.g. Word Document, Notepad, Word Webpages etc). Just give it a name in the Doc Label box and search of it using the "Browse" button. After that, upload it using the "Submit Document" Button at the bottom. It should tell you in green writing on a refreshed page that your document was uploaded successfully. From there, click on the "Story" tab and click on the "New Story" link in the top right side of the screen. There, I guess you classify your fic (rating, genre etc.) and then you add you uploaded document to it. I'm not too sure on how the "New Story" feature works coz I've already got a story up on fanfiction and I havent had a new one up since they changed the layout of the place!
Hope that helps...if it does at all! But you have to write your story in a word processing program on your computer FIRST before you can upload it to Fanfiction. You can check out what file types the site excepts but I think it the majority of the widely used processing software programs