A good first line does not a good novel make, but a bad opening could kill a great novel before it's had a chance.
So what makes a good first line?
There are a few key points:
Change - if this is a day like any other, why are you writing about it? All stories are about change, so if you want to get your reader hooked right away, get straight into it!
Question - you need your reader wanting more. If you can make your first line raise questions that they cannot live without knowing the answer to, you can be pretty sure they'll keep reading. Of course, what constitutes a gripping question will vary depending on your readership. A computer geek is unlikely to care if the question is the heroine wondering how she'll ever find love - equally, a romance buff cares little why the Republic of Somewhere Alien is about to implode.
Surprise - for extra flavour and style, if you can surprise your reader in so few words, with something unexpected or out of the ordinary, they will want to read on to explain the mystery.
Some examples of awesome opening lines:
It was a bright cold day in April, and the clocks were striking thirteen.
George Orwell, 1984
Mira was hiding in the ladies' room.
Fay Weldon, The Women's Room
My name was Salmon, like the fish; first name, Susie. I was fourteen when I was murdered on December 6, 1973.
Alice Sebold, The Lovely Bones
We slept in what had once been the gymnasium.
Margaret Atwood, The Handmaid's Tale
Think about which of the key points above each of these meet. It's a great exercise to randomly pick books off your shelf and read their first lines.