Let's start at the very beginning
In memory of Christopher Plummer, I'm starting today's post with a quote from The Sound of Music:
Let's start at the very beginning,
A very good place to start.
Where you start your novel is a crucial decision - and it's not at the very beginning of your character's story as you might think. I made that mistake with my first ever attempt at a novel. Don't laugh, but my opening words were "I was born on a day when..." In case it's not already clear, your story does NOT start on the day your character was born!
The general advice for writers is to start at the moment of change for your character. That means that your story shouldn't include a long introduction. Yes, we need to see your character in their 'ordinary world' to understand what their current situation is and what needs to change, but too long in that world without seeing them challenged, and readers will lose interest.
The "moment of change" is the inciting incident of your story, the moment when the path they were on changes. Readers are along for the ride of this new journey, so don't spend too much time in their "before" world. (And just because Elizabeth Gaskell's North and South had a lengthy introduction doesn't mean that you, writing for a 21st century audience, can too!)
It's also important to start at a moment of action. These opening pages are your one shot at getting a busy editor or agent to decide whether or not they want to read more. Most agents only ask for the first ten pages of your manuscript - so you cannot wait to get the story going on page 30! Likewise, readers today have short attention spans, and twenty million other things they could be doing with their free time, so you have only the length of the free 'Look inside' option that Amazon and other retailers offer, to get them to decide whether or not to spend money and time on your book.
Starting with action doesn't mean throwing the reader into an action-packed scene in which bullets are flying and the poor reader has no idea what's going on. It means having an active opening, in which we learn about your character as we see them interacting with the world around them. No navel-gazing, no studying themselves in a mirror so the reader can "see" them, no lengthy descriptions, and no flashbacks - those are all passive openings.
If you're convinced that the flashback or back story is absolutely crucial and must be included in the opening pages - then maybe you're starting your story in the wrong place, or telling the wrong story!
Many beginner writers figure their way around this is to open with an active, dramatic scene - which then turns out to be a dream. Please don't! What happens is that the readers builds up an expectation from the opening scene of what your book is about. When that all turns out to be a lie, readers will feel as if you've betrayed them, and the chances are slim they'll trust you enough to keep reading.
So what should you include in your opening scene? The first pages of the book should give readers a feel for the following:
the book's genre - is this book light and humorous, or serious and dark?
the main character
a goal for the main character, even if it's a small one at this stage, rather than a big story goal
a hint of the conflict or issues that will be faced in this book
what's at stake? What does the main character have to win or lose in this book?
If your opening scene includes all the above (and the writing is polished and largely typo free) you'll have set up the reader's expectations in a way that will guarantee they keep reading past the few pages.
If you're wondering whether or not to include that back story or flashback you're so attached to as a prologue, read my post To prologue or not to prologue.