A story needs a beginning. I think beginnings are the easiest.. Then it needs some characters, some background, a scene. Give that person a goal, a direction, a purpose. Then you need to do the hard bit and figure out how this character goes from A to Z.. and there may be a lot of plot twists, or timelines, or crazy stuff going on.. whatever you want to write.. Its up to you. Let your fingers type it, keep going.. read it again the next day.. Improve it, throw out chapters and rewrite them until the story is what you want. Some people find all of this easy, some struggle with it. Like playing the guitar, it takes years to master, but everyone starts at the beginning.