In our everyday conversations, we begin a story with the most exciting part, then back up to fill in the details. The same is true in good writing. Begin with the exciting part, then back up to fill in the details.
To get started, write whatever you feel most strongly about or the part of the story you most want to write. This is your emotional trigger. Write forward and backward from your starting point. Don't be concerned with what belongs or doesn't belong. Don't be concerned with the order in which the different parts should be presented. Rewriting and editing come later. In writing the story, you will discover how the story should begin for the reader.
Sometimes the emotional trigger is not clear—you just have a vague desire to write about something that's on your mind. In that case, start with whatever you have—a bit of dialogue, a particular setting, a distinct character, a what-if question or an action scene.
The point is to begin. What your write may seem dull or muddy or just plain dumb. No matter. Sometimes we need to write our way through the lousy to get to the good stuff. So begin.