First lines are important. They surmise everything about your writing style and hint at what’s to come in the book. At least, they should.
I’m unsure about my first line. It doesn’t do that and it should. It’s boring, sterile, not as sharp and witty as I want it to be. I’ve already changed it more times than I can count and I’ll probably change it a dozen more times until it’s just right. Along with the first chapter, which needs a rewrite itself!
How do you know when you’ve got it? You’ll know. It’s like being slammed with a mental two-by-for. You sit, stunned at your own genius (and perhaps not undeservedly), because your joy at this line diminishes any concerns you have about its appropriateness. When it’s good, it’s good.
Let’s look at some opening lines.
“The man in black fled across the desert and the gunslinger followed.”
---THE DARK TOWER: THE GUNSLINGER by Stephen King
Whether or not Stephen King is a good writer is irrelevant. It’s a damn good opening. In that one sentence, you learn everything you need to to ready yourself for the books. The man in black is fleeing. The gunslinger is pursuing, something that resonates through the rest of the series.
"It was a bright cold day in April, and the clocks were striking thirteen."
---"1984" by George Orwell
The clocks are striking thirteen. The perfect description of the skewed work that follows.
"All this happened, more or less."
---SLAUGHTERHOUSE-FIVE by Kurt Vonnegut
If you've read the book you'll understand how much this sums it up, prepares you for what follows, whether or not it happened.
"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents."
---The Call of Cthulhu by H.P. Lovecraft
What can be so horrible that it's better to not understand it? Read on and you'll find out it's the Great Cthulhu, thank you very much. All hail Great Cthulhu!
This is only a small sampling, but it shows how important that first line is. This is the start to your story. It doesn't have to be memorable, but it should tell everything that's going to happen even though it can't come right out and say it.