One of the most important decisions a writer has to make is where to start the story. Start too soon and it gets boring waiting for important events. Start too late and you have to go back and explain too much in backstory. Like Goldilocks we have to find the spot that's juuuust right.
Usually, that spot is just before a huge life-changing event for the protagonist. In my book, Genetic Attraction, the action begins just about an hour before the hero tells the heroine he has a male lover, dashing all her hopes for being with him, or so she thinks. In the prequel to Genetic Attraction, that hopefully you'll see next year also, the story starts just as Jake meets his beautiful lover and his life changes.
Once you know where to start, there's the issue of the first line. Will it grab the reader? Maybe go down in history like "It was the best of times. It was the worst of times." or "Who is John Galt?" That might be too much to hope for in every book, but authors still try to grab their reader's attention in the first paragraph. I love to start with dialogue. I think it plunges the reader into action and connection with a character. It suggests that the book will be be more about people than description.
The first line of my current work in progress is, "Did you come, honey?" Guess what that book's about?