Intuitively, writers start writing at the beginning of their story. After all, stories have a beginning, a middle and an end. In reality, your story should begin in the MIDDLE. To be clear, this isn’t about beginning the story in the middle of some action beat. Yes, the latter is a tool that gets the reader engaged early in the story. By “middle” we mean overall story.
There should be enough back story equal to the story a writer puts out. In other words, if you plan to write a story about a handicapped woman looking to climb Mount Everest, you may elect to start the story with the climber getting her gear ready just prior to setting off on her journey. In reality, there should be enough backstory that supports why the woman is attempting this climb. In many stories, the back story comes out as the story develops but not always —and that’s okay. At the very least, you (the writer) should understand all the details related to the backstory. For instance, the woman may have been ridiculed throughout her life for her handicap. Perhaps her sister was an expert climber and your protagonist had to endure years of seeing her sister getting all the accolades that come with her climbing wins.
Whether you elect to disclose some or all of your backstory to your audience is up to you the writer, however the reader should come to understand the reasons why your protagonist does the things she does.