Well, we can't really tell you what to write. You should come up with that on your own. Just write what you want. Obviously try to follow the standard rules of the English (unless you're writing in another language, then follow the standards of that language). Also, research research research! And keep the characters as close to canon as possible. There's not much else I can tell you other than that since I know very little about 1D.
As for fanfiction, I have written some, but none of it is 1D.
I don't see why you're asking for help with fan fiction. If you're a fan of something to the point of wanting to write for it, you probably already know what you want to write about, and you're pretending you don't because you're unsure of yourself. You don't need to worry, though. As long as you like your ideas, and write them with every ounce of emotion they bring to you, someone will surely love reading them. I promise.