Whether you are looking to snare an agent, a publisher, or a reader, your first line is all important. Leaving aside the cover and back-cover blurb, the opening line delivers the first impression of your story, and it needs to be a good one.
So what constitutes a good first line?
A good first line sets the tone for what will follow. For example, in a fun, noir-inspired mystery, the first line ought to be snappy and hard-boiled. Janet Evanovich's One For The Money (the first in her bestselling Stephanie Plum novels), opening line is:
There are some men who enter a woman's life and screw it up forever.
The tone is perfect. Also, while the front cover and blurb state that the book is a mystery, the first line broadcasts the book's humour and its other major theme – (troubled) romantic relationships.
Similarly, in a lyrical, literary work, then you'd expect the first line to be poetic. For example, the first line of Gabriel Garcia Marquez’s Love in the Time of Cholera:
It was inevitable: the scent of bitter almonds always reminded him of the fate of unrequited love.
A good first line needs to hook the reader. Now, a lot gets made of this point, but it’s worth remembering that there's more than one way to grab your reader. If your prose is beautiful enough, then a gorgeous turn of phrase or intriguing image might do the job.
Also, it's the kind of advice that leads people to add false conflict into their opening lines. To give a (made up) example:
The bomb went off, flattening the downtown district in an instant.
Now, putting aside the merits (or otherwise) of this opener, if it is then followed by:
Across the state and unaware of such things, Mary Smith, watered her begonias.
You may have a problem…
If Mary and the bomb are utterly unrelated (and remain so throughout the story), the conflict promised in the opening line is false.
Think of your first line as a promise. A person who is promised bombs in the very first line is unlikely to be thrilled by the following 80,000 words of quiet reflection on the nature of gardening.
Equally, a person who wants a cosy mystery about the begonia thief may be put off by a whizz-bang, punch-you-in-the-face opener.
Of course, if your book is full of such juxtapositions of high, national drama, and small-life, personal detail, those first two lines might be perfect. As always, you just need to think about your intention, the words you’ve used and the effect they create.
As with the rest of your opening scene (which we’ll come back to another time), it's a good idea for your opening line to actually be the opening of your story. Or, to put it another way, to start where the trouble (ie. your story) actually starts.
For a great example of this, see the first line of Richard Matheson’s I am Legend:
On those cloudy days, Robert Neville was never sure when sunset came, and sometimes they were in the streets before he could get back.
It’s perfect. You have a character in trouble and a question - who are 'they'?
First lines deserve your love and attention and, most likely, you'll rewrite them several times before getting them right. Personally, I can't know if a first line is 'right' until I've finished the whole book, other writers can't get started unless they've penned the perfect opener.