Download CSIDE. There are several tutorials built into the app that will walk you through everything step-by-step.
As well as what @Sinnie said, look at some visual graphs of how choice stories are organized to find one that matches your story idea.
Lastly, try playing some (completed) choice games. Make notes while you play of how the story unfolded based on your choices and inputs (such as telling the game what your name is) so that you can start to better understand what separates interactive fiction from “regular” fiction.