First off is know the audience you are going to write the book for. Young adult, Adults, or middle school. Then know what sub-genre you plan writing. If you plan on writing a historical book, you'll have to heavily research the time period that you plan on writing.
The second thing you'll want to do for your story is plot the actual storyline. Every author has their own way of doing their storyline. Some like to use the old English literary style that they taught us back in elementary school for writing stories, a plot flow chart.
Another way to plot is by making a big sheet of and making a nine box flow chart of the sequence of events to happen.
Some will even do it the old school way and a make flashcards, and post them on a board so they have something to refer back to as they are writing the story. I used to do it this way.
I normally write all of my stories in a word document, but this year I'm going to try something different, I have heard a lot of who use the writing software Scrivener for drafting their manuscripts, then exporting them to word to fine tune it. Since NaNoWriMo offers a discount to NaNoWriMo winners, I'm going give it a try. It's suppose to help writers, and it allows for us to to move chapters around easily, make notecards for plotting, and export into formats for self publishing.
When I start using it for NaNoWriMo I'll make a post about it with some pictures.