Fan fiction, often called "fanfic," is a mostly online genre of amateur work generated by fans of print and video popular fiction. Fanfic writers use characters, settings and story lines owned by authors and publishers as the basis for their own stories. Authors and others are often displeased by this use of their creations, and fanfic writers and publishers have been sued in the past. To prevent this problem and show respect for fictional property owners, most fanfic writers place a disclaimer, often humorously written, at the beginning of their published stories.
Determine whether your fan fiction is on a list expressly disallowed by the creators of the world you're writing in. Popular writers like Laurell K. Hamilton and Anne Rice will sue fan fiction creators who use their characters and worlds. Some writers also require that you expressly assign ownership of your fan fiction to them in your disclaimer. Write the author if you aren't certain.
Begin your disclaimer by clearly stating whose world and characters you are using. For instance, "This is a work of fan fiction using characters from the Harry Potter world, which is trademarked by J. K. Rowling."
State that you do not claim ownership of the characters and world that you are borrowing. "Both Harry Potter and Ron Weasley are characters created and owned by J.K. Rowling, and I do not claim any ownership over them or the world of Harry Potter."
State that the story and situation you are creating are a work of your imagination and you do not ascribe them to official story canon. Further state that this is a work intended for entertainment outside the official storyline owned by the author. "The story I tell here about Harry and Ron is my own invention, and it is not purported or believed to be part of J.K. Rowling's story canon. This story is for entertainment only and is not part of the official story line."
State that you are not profiting financially from the creation and publication of this story.
Thank the story owner for his contribution to the world of entertainment. "I am grateful to Ms. Rowling for her wonderful stories about Hogwarts, for without her books, my story would not exist."
Go back through your disclaimer and make it funny if you wish. Provided you make the statements outlined in the previous steps, you can create your disclaimer to have any tone you wish.
Note
The first step, identifying the story and the author, is a crucial courtesy. Many authors will not read fan fiction about their characters because some authors, including Marion Zimmer Bradley, have been sued by fanfic creators for stealing story ideas.
If you plan to profit from your fan fiction, be warned: Writers are much more likely to sue you and win. Free sharing of fan fiction is usually covered under fair use copyright law, though that may change in the future.