Depends on your inspiration, as that is the one factor you cannot control and when you lack it, then you get what we call the "writer's block" and that is getting a bit to me now. Aside from inspiration you also need the ability to present your story well... Know how to make it interesting for the reader. A good plot line but poorly written out can drive readers away, yet a crappy plot but written well can attract the reader more at first... Of course if they think things true they can be disappointed anyway.
When it comes to the mastery of the language you write in (and I recommend to use your first language only and leave other languages, no matter how well you speak them to translators), you must at least know what kind of idioms are common and have the basic knowledge of all the rules, however I do recommend not to focus too much on that, and leave that for when your first draft is done. Being too picky on that will ruin your story. And of course, always let somebody else do the final spellcheck as you always have a blind spot for your own language errors, both on spelling and grammar.
Of course you first need a global plotline. A synopsis, an extremely short version of how the story should go. From that synopsis you can think about the details and slowly your story will take shape. You also need to have a good proper view on your personalities... their strengths and their weaknesses. Nobody wants a perfect person, but a person everybody hates, well it can work, but only for specific roles (villains in particular). When you have a villain you need to give that villain a good background story. Why is the villain "evil"? A villain who is just evil for being evil's sake won't work. Voldemort in HP worked so well because he has a really worked out background story.. a story that has more depth than Harry's own. Make the reader understand why the villain is a villain... Note, I said understand, not justify, although in some stories the actions of the villains are justified, but then you are dealing with a villain who will eventually turn out not to be a villain. Those are very hard to set up, as you then need to fool your readers. Distracting the readers yet giving them tons of hints... that's where the hard part lies.