After figuring out what POV to write your book in, you have to figure out where the story starts. This is one of the hardest decisions in writing and also the most important one if you want to capture the readers attention. Luckily there is only one place to start a story, with several variations.
THE TRIGGERING INCIDENT
People at rest tend to stay at rest until acted on by an outside force. When your main character hits something that changes his life that starts the story. The beginning of the story needs to take place somewhere near this incident. How close depends on the type of story you are telling.
THE CHARACTER INCIDENT
If you want people to relate to your main character the story has to begin shortly before triggering incident and at the decision that main character made that brought him to the incident. In THE PIZZA DIARIES retitled AN EXTRA TOPPING OF HORROR I originally started at the triggering incident where my main character saves the damsel in distress. It seemed logical, but he really didn't have any choices to make unless he was a true bastard. There was a naked woman dazed in the road. Unless he was a total psychopath he had no choice but to help her. As a result for the entire first chapter he was pushed along by the events that happened. That shaped the readers impression of him as someone who just got pushed along through life. So I had to start earlier.
I had to start at the decision that brought him out there in the first place. An order came in from a man that he had a bad history with. He was given the choice of passing it to his co-worker, Kyle, who had plans that would be ruined by taking the order, or taking the delivery. So he had make a decision that showed he was a half way decent person. Not terribly noble, but considerate of his co-workers. It was the type of choice that people are faced with all the time so the reader can relate.
By starting with an ordinary decision the reader can then see how the same decision making process plays out when he is faced with really extraordinary circumstances.
THE ACTION INCIDENT
If your book is focused on the action then the story starts right after the triggering incident in the middle of the action it caused. Then it backs up to the incident, preferably through dialog. This lets the reader know to focus on the action.
The main motivator in porn and romance, someone has a desire for something they may not even know what it is. The trigger is when they see a chance of having that desire satisfied. If your story is about someone's quest to achieve something then you should show the longing that was inside them before the triggering event.
This example will seem strange but bear with me (or if you are naked, bare with me):
2001: A Space Odyssey. The story starts with the caveman “Moon Watcher”, um, watching the Moon. He longs for humans to be able to great things like find a tree high enough to touch the Moon. He also dreams of a time when humans will be safe from predators and well fed. The triggering incident comes when the Monolith lands and transforms them into a species that has the tool, a brain, to satisfy that longing. 10,000 years later his dream comes true.
THE INCIDENT ITSELF
If you are really good or lucky all those events happen at the same time. The incident forces the main character to make a decision and leap into action that satisfies a longing. Chances are that won't happen. Rather than try and force the incident to fill all these things in the first page (something that is really obvious and usually awful) it is best to find the part about the incident that will shape the impression you want the reader to have.
While writing this I have an awful nagging in the back of my mind that I am missing a key part of the triggering incident, but I can't think of what. Somebody enlighten me in the comments so I can do a face palm as soon as I read it.