Hook Your Reader Hard and Fast

Let's talk about story Hooks, since everyone struggles to write these effectively.
Regardless of your genre, a good way to start a story is to reveal a situation that is about to explode.
Most books start on the brink of catastrophe or a character's catharsis (read “awakening”). Events are literally about to go wrong, and your readers are going to have to witness the devastation and recovery/enlightenment. You need to be sure which is the case for your story to truly create the best opening.
That’s what we call the story's Promise: to take the readers on a ride and deliver them, along with the character, to a new location and/or state of being.

Open with a Bang

Ask yourself this: “Is the character's internal or external world about to explode, and how can I best represent this building tension in the opening lines?” That explosion can vary from literal world destruction to a simple marriage argument (when are those ever simple?), from a lady’s dog about to stray into the road to a confirmed bachelor about to meet the woman of his dreams (which might “destroy” the world as he knows it).
Jessica Page Morrell writes in her book “Thanks, But This Isn’t For Us: A (Sort of) Compassionate Guide to Why Your Writing is Being Rejected”:
The best beginnings are like forces gathering, about to be unleashed on the reader. With the first words, the writer establishes his credibility, introduces viewpoint and voice, and makes the reader care about people and the story unfolding. Obviously this is a tall order for a few sentences or paragraphs to accomplish. Also, since fiction and memoir are based on adversity, typically an opening introduces a character or person under stress and they story world staring to tilt off balance.

Rarely do we read stories that start with something going right. There’s no tension when the world is perfect. The exception would be a character experiencing a lot of happy “rights” in order to build tension because something is about to bring that character’s world to a crashing halt.

Hook Your Reader Fast

A Hook typically involves the unique problem/situation that your character must address in the story. Maybe it’s a nun who gets pregnant. That’s a situation that MUST inherently, physically change with or without the character's consent. Or a race car driver who nearly died in his last race. There’s no inherent change for him, unless we see him get back in his car. Then the reader will suspect something is about to steer off course.
A Hook may only hint at the unique problem/situation. That means a story of a confirmed bachelor doesn’t open with him showing resistance to marriage, but him not taking on his sister’s puppy when she unexpectedly has to move abroad. That shows a layer of his commitment issues without throwing too much in the reader's face too soon. His self-centered nature is revealed just enough in the opening that we know he is going to fall. How he falls and the unique turns he takes along the way to change make up the rest of the story.
Basically, when you start a story you want to capture the readers’ attention so they ask, “What will happen next?” In fact, every time you end or open a chapter you should be creating that response for the reader all over again.
TIP:  Hooks are not just for opening the book. They are for opening EVERY chapter, EVERY scene. You can also use them to end scenes and chapters to get the reader to turn the page.
Morrell sums this up well:
The best openings of a story, novel, or memoir are contagious—they make the reader yearn for more because you chose the best words at the best moment to launch the events that follow while raising questions that demand answers.
 
Your Hook doesn’t necessarily have to be the first line of the book, but nowadays you might as well put it there, considering the immense competition and the little time your story’s given by an agent or editor. My advice: go for the jugular fast, but keep it natural to your character and story.
Here’s a good example from Kristen Higgins' Catch of the Day: “Falling in love with a catholic priest was not my smartest move.”  We are intrigued to know how she fell for him and what she’s going to do about it. Moreover, we want to know what HE’S going to do next. Will he forgo his orders to love her in return, maybe even marry her? Or will he break her heart?
Remember: raising questions that DEMAND answers creates reader engagement.