Welcome aboard. It's easy. Just go to the Fan Fiction Alley.
If you want a new topic at the top of the board you will see NEW TOPIC in blue. Just click it and the post will open up for you to write.
If you want to reply to a thread that is already there just hit reply. If you want to quote the person just hit quote in their post.
And Fan Fiction [not copyrighted] can be uploaded too on that forum
Enjoy. And if you run into trouble just ask.