In other words, Why now?
It's important to think, "Why now?" What is it about this day, this minute, that is special? When did the dominoes start to fall that would eventually lead to the conflict, climax, and resolution of the story? The time-honored example is the line, "Where's Papa going with that Ax?" from CHARLOTTE'S WEB, which not only hooks the reader immediately into the action, but clearly shows that if this moment on this day hadn't happened, the rest of the story would never have taken place. Wilbur would have been killed as a runt and that's that.
Sometimes it's important, as the writer, to go back and write about how Fern was a good girl who lived on a farm with her mother and father who raised animals and participated in the county fair. We could delve into Fern's dreams, her secret wishes, her want to have a friend or a pet to care for and love as her own. Perhaps we could start with spiders in the barn, or rats, or the farm animal community who live together and quietly do not speak of their eventual demise. (Oh, wait, that's BABE. Or maybe ANIMAL FARM...) Anyway, we could start there, but while that might establish character, setting, foreshadowing, etc., it doesn't set up the story that happens right at that moment when Fern's and and Wilbur's and Charlotte's life changes forever. (Ever notice it's called Charlotte's Web and doesn't even mention Fern or Wilbur. Huh.) That is the moment where we want to begin.
My kids and I borrowed WALL-E from the library and again I thought about all-important beginnings. The little robot going through its clean-up routine all by itself and dreaming big dreams of romance and the stars until that fateful day it found a tiny plant growing in the earth, the spot of red laser light appearing, and his world forever changing with the arrival of Eve. It had to happen this way, bringing us into the Moment The World Changed Forever. This is what the reader needs ASAP in order to get the gist of the character motivation, the momentum of the story, and follow the tale for the many hours/days/years they might invest in this character, this world, this author, for the first read and beyond.
Where do we start? Hook 'em hard and introduce the Moment The World Changed Forever as fast as you can.
At least it's a very good place to start. ;-)