Wait... wait, wait, wait.
You want to write a FAN FICTION, and don't know how to start it?
Jesus... that sounds like me once upon a time.
I take it, you didn't write a carefully planned plot outline? You just have everything in your head, right?
If the answer is: YES, then... BACK THE HELL UP!
You gotta write those things down, man. EVERYTHING. Starting from character profiles(if you're using original charas, that is) and ending on the plot outline.
You can't just write from your head, because that's not how this [parasitic bomb] works. Not even in fanfiction.
If you do, however, have written a plot outline already... then just start writing. There's really nothing that stops you.
But as I said, I don't think you have it written out.
Tell us what this "story" or "fan fic" is supposed to be about? What series?
C'mon.