Create an outline for the story you have in mind. Write it down by chapter so that you have a basis to work from. Make sure you write the plot down and any other ideas you have for the story. In addition to the outline, create a separate page for characters in the story.
Start slowly and write one section at a time. If it seems too overwhelming to write a whole book, break it down into parts and work on one part at a time. When you are finished with one, move on to the next.
Write every day, even if it is just a little. If you are blocked and can't continue writing your story, then get a blank piece of paper or a blank computer screen and write down anything that is on your mind. Even writing the same word or phrase over and over will help to spark some ideas.
Set the story aside for a few weeks once you are done writing it, and come back to it later. Look at it again with fresh eyes and you may come up with some new ideas or plot twists to make it better.
Take your time. It takes skill and patience to write a book, so don't try to finish it as fast as you can or it will most likely be no good.