Advice from many good writers that I've gathered over the years. 

THE HOOK

The opening paragraphs of a book are often called the hook—the opportunity to hook the reader on the book. Most successful openings include a change in the status quo—something is happening that makes the reader want to read further and find out how the characters deal with it and the outcome. The character or characters must be strong enough that the reader cares about them right away and wants to keep turning the pages. Pick an interesting event or create one and start the book with some kind of action and, possibly, dialogue. Dialogue should be short and snappy. Introduce at least one of the main characters and show the scene.

A hook should have three things: 1) where (place and time), 2) what's happening now, and 3) whose point of view. Keep the beginning short. Leave past history out of the hook and the opening scene. Save explanations, except for the absolute minimum the reader needs to know at this point, until later. The part immediately following the opening scene usually gives some background or history, but the beginning should be so compelling that the reader resents being taken out of the immediate scene.

Don't start at the beginning.  Nothing is happening there.  Start in the middle of things.  In media res. Start as late as possible; as close to the end as possible.  The later you can start without losing your readers, the better.

The beginning of your story must do at least the following three things:

1.         Get your story going and set the tone

2.         Introduce and characterize the protagonist (the first character you introduce should be the protagonist)

3.       Engage the reader's interest!

The opening scene can also create mood, introduce the narrator or narrative voice, introduce other characters, etc.

The beginning of story lays down the promise for the rest of story and you have to be willing to live by the rules you set. So be careful to open with the right promise. Don’t promise humor and deliver tragedy.  Don’t open with lyrical, poetic writing unless that’s the tone of the story.

 Backstory

A backstory (also back-story or back story) is the history behind the situation that’s current at the start of the main story. Backstory is used to lend the main story depth or the appearance of reality. A backstory may include the history of characters, objects, countries, or other elements of the main story. It can be revealed in flashback, dialogue, or exposition. Scatter the information through the story as it’s needed. Don’t tell the whole story at once.

Avoid using backstory in the opening scene. Let the reader wonder for a little while. Drop hints to generate interest, but don’t stop the action to give explanations.

Backstories are usually revealed in small bits as needed as the story progresses. Sometimes the writer creates an involved backstory for herself but only uses a small part in the story. A notable exception is George Lucas, who made three movies out of backstory. After he wrote the original Star Wars movies, he went back to explain where the characters came from. The backstory became The Phantom Menace, Attack of the Clones, and Revenge of the Sith,