Now,for it to be a true fanfic....you must have an anime to base it off of....which makes it easier to write...because you dont have to make up a whole background and things like that....
so step 1....Pick the anime/manga to base it on
step 2 find out what format you want
what i mean by format is some people make their fanfic a poem....and some just action.....or you can go with the ever popular...manga format, as i like to call it...in the manga format you must center all your work....words are in "quotation marks" and actions are taken care of with *asteriks* ....thoughts are with italics and important words are BOLDED and capitalized.