It may seem obvious where you think your book should start, but if you take a look at it from a reader's perspective, will the beginning seem too slow? Sometimes it's better to start your story in the middle of the action. For example, if your protagonist is kidnapped in the beginning, start the very first line with that. I've seen too many writers introduce their protagonist's friends in the first chapter, including how they go to a party and so on. Instead, cutting out all that extra fluff leading up to the kidnapping scene will pull the reader directly into the action. If any of the fluff was necessary information, have your character reflect back on the events after being kidnapped.