I'm sure you learned this one way back in grade school, and you'll recognize this structure from all forms of modern day story telling. The three act structure consists of braking up your story into three basic parts of your story, the setup, conflict, and resolution. Each one of these elements are just as important as the others. Today we're going to take a look at the setup.
The Setup- this is your introduction of the books tone, protagonist, other main characters, and setting. Some stories jump right into the action, while others take a little more time and start a bit slower. The important thing is that you want to give your reader a reason to keep reading.
I have learned from hundreds of hours of experience that the same thing is true of public speaking. You have about 5 minutes for your audience to decide if they will continue to listen to you or not. I don't know how many pages that would equate to in a book, but the point is to introduce your readers to something that draws them into your material and gives them a hunger to keep reading.
Next time I'll write a little bit about conflict, but in the meantime, here is an excellent article on the basic Three Act Structure.