Read the story. It will be very difficult to summarize a story without actually reading it. So crack open your book, or plug in some headphones and listen to it on your iPod. Don't always trust the internet sites that claim to summarize books, because they aren't always accurate.
As you're reading you need to keep in mind what the central idea of the story is. For Lord of the Rings, for instance, the central idea might be something about how the power of greed (i.e. the Ring) is a strong force for evil, or even the actions of one insignificant person (like a hobbit) can change the world.