The first pages of your book are important. The first paragraph, the first line even. You need to grab the reader by the throat from the get go to make sure they keep reading to the bottom and turn the page.
But you don't need to give away the whole book on the first page.
So many contests out there ask for the first 100-500 words of your manuscript and judge the book on those few words. And yes, they are important. But I don't believe you have to give away the entire plot or even that much information in the first page.
A reader will be able to tell within a few lines if the writing is good. Certainly, that's something I look for in an opening page. If there are spelling or grammar errors, I will probably pass on reading any further. If the prose is stilted or repetitive, ditto. If I'm dumped in the middle of a huge action scene, with no idea who the heroes and villains are, I'm probably going to put down the book.
So what entices me to read past the opening lines? To take the book home from the library rather than back onto the shelves?
Voice, for one. If I like the way the character comes off the page, I'll go with it, regardless of what's going on. If I'm interested in the situation the character's in, I'll read on. If I'm intrigued by something I don't fully comprehend, I'll keep going until that curiosity is sated. And usually by then, I'm captured.
It's very rare that I'll put down a book after a first page. In fact, usually I'll give it a chapter or two. I hate to quit reading something and will usually keep going even if I'm not liking it especially. I've been surprised often enough by books that are boring as hell for 3/4 of their length and then became utterly compelling in that last 100 pages.
Maybe I'm unusual...
How far into a book do you tend to get before putting it down?