If you're at that stage then you're thinking about beginnings. Or in the case of some writers you may be thinking about the middle or the end - but for most writers at this point you're wrestling with the beginning.
So you need a strong beginning, something to pique the reader's or editor's interest and they'll want to turn page after page until the book is done. All that hinges on the beginning - at whatever stage you think about such things.
So, how do you hook 'em?
A question can work well. I've seen that used many times with great results. Pick up a few books, read the first page and frequently you'll find that question, the one that makes you wonder - and enough information to catch you, the reader. After all, if the question engages you and makes you wonder about the answer you'll no doubt do a bit more reading before thinking of putting it down. And if you read past the answer you're probably going to buy the book. That's what you want in your own book.
Want to catch attention? You might try throwing your main character into the middle of a terrifying situation. "The forest trembled, the fire erupted around him with a force that sucked the oxygen from his lungs and all he could think was, I didn't do it!"
Your first words can be someone else's. Dialogue is a great way to open a story and catch a reader's attention. People love to come in on other people's conversations. Ever picked up a weird tidbit when passing strangers in public? "Is it just me or has the entire world gone stump stupid?" Sherrilyn Kenyon's "No Mercy". Now there is a line to keep your editor or reader reading.
But you don't always have to depend on some kind of action to get things rolling. How about "Six minutes from now, one of us would be dead. That was our fate..." Brad Meltzer's "The Book of Fate". Now that's a problem! How can the book continue if that's the case? Have to read on to find out.
So, you have that idea for a book swimming around in your head? Pluck out a bit that's sure to grab and start your story there. Grab your reader from the beginning and don't let go.