1. A driving scene.
Have you ever read those books that start off with a slow description of someone's whereabouts or circumstances? What about the books that start simply with a sitting down scene and almost no dialogue or action? I'm afraid you can only get away with it in classic literature. Times have changed, and if you don't have your reader hooked by the first three or four pages, it's often a lost cause.
When starting a novel, start it with a driving scene - something that instantly triggers the action that is going to carry the story. Gustav Freytag, a German playwright, used to talk about the "inciting moment". Start with an "inciting moment". Does your character desperately want something? Introduce this in the first scene. Does something terrible happen to your character? Introduce it in the first scene. If it feels too soon, you can always tell the back story in an extended flashback and work up to the same scene later in the book. Does your character fall in love? Introduce this in the first scene. Start with dialogue or action. Paint the pictures vividly. Don't just describe vaguely or lazily. "Write like a movie." Would a movie start with a sitting down scene?
2. A driving character.
There is nothing worse than reading a book where the characters feel one dimensional. Don't kid yourself - you can put all the action in the world in your novel, but unless the reader cares about the characters, they are not going to read it. People watch the news all the time, but what happens in the Middle East fails to touch their hearts because they don't care about the people. Unfortunately this is human nature. Make your reader care. Invite them into your character's head, heart, and past. Show them your character's deepest desire, right away. If the reader doesn't ever find out, some character development is lacking. Show them snippets of your character's past. Why is that particular person a killing machine? What made them like that? Why did Anthony enter the army? Why did Sarah give her child away? Explore the character's motive. And once you've explored that, bring it right into the first scene.
From the very start of the book, the reader should connect with your characters. If they don't, they will not be hooked. You must give just enough of your first-mentioned character's motives, history, and desires for the reader to be engaged. This shouldn't take any more than a paragraph - two, if you're being liberal. But it shouldn't take too long. Just get to the point. Tell the important information. Summarise, like you would summarise your own desires and motivations if you were being interviewed. "Write like a movie." Would a movie director take a long time to show you what the main character desires or believes in? Of course not.
These are two basic principles that I believe will help an author write an exciting first scene. These principles make all the difference between a book that someone can live without, or a book that compels the reader on. It may even make all the difference between whether you get a writing contract or not.