Read the draft. You will reread your book maybe fifty times by the time it’s ready. Get over it.
1-Beginning. Does your story start right in the middle of the conflict? Don’t beat around the bush, engage the reader. What’s at stake? The reader needs a hook (not necessarily the climax of your story, but whatever will engage the reader). You need to make them curious enough to read the first few pages and empathize with your characters.
It's okay to decide on the first chapter last ;)
2-The middle. Every single scene and dialogue needs to move the story forward. The reader needs to learn something either about the plot or about a character. If the story does not move forward cut the scene/chapter out! You will learn to recognize that slight itch in the back of your brain, that little voice telling you that maybe a chapter (or a paragraph) doesn’t need to be there. They don’t. Take them out, now. Like… now. I can almost hear you say, “But I really loved that sentence! That line was so funny…” Yes, but that does not justify the scene around it. Save the good lines for an awesome scene that won’t bore your readers to death.
3-The end. Did you tie every loose end? If you start a side plot make sure to close it. You don’t need all characters to marry, die, or save the world but each plot is like a bullet you shot: it goes up, reaches a climax and it has to comes down. If you don’t close some side plots take them out! A great ending is where the skill of the writer comes into play. I love catharsis (the process of releasing and providing relief from strong and repressed emotions). You wrote a whole book about a conflict, well let me enjoy the resolution! If your book is part of a series, it still needs its own wrap up!