The Three Steps: Explore
Explore Your Story
The story you think you are going to write down often ends up radically changed by the process of writing it and working on it. This is the fun part of writing, the wild act of discovery that pulls you into the story.
Plot
You may have no idea what your plot is going to be when you start, but writing it down will let you immerse yourself in it. Or maybe you know exactly what ending you are moving towards. It makes no difference, because there's only one way to reach the ending in any case: just write your way towards it!
Challenges
Sometimes, things get hard for your characters. That's good! Sometimes, things get so hard that they get stuck. That's not so good. But you have a secret power: you are in complete control of the world of the story. You can always cheat just a little bit to make sure that things are only ever exactly hard enough that your characters have an adventure without getting stuck.
Twists
Sometimes, the characters or the plot throws a surprise that even you, the author, did not see coming. Good! If a story isn't fun for you, it won't get finished, and if it's not exciting to you it may not get read. If it works, then run with it. And if it doesn't, no big deal, you can always change it.