Here’s a great post about what NaNoWriMo is great for, and what it is not. It will help you create a first draft and get down unexpected ideas. A draft like that will need many revisions. Leave your finished work alone for a while at the end of the month. Then go back and start the work of reshaping, adding, and cutting that will make your story work.
Here’s a terrific post about how to revise a novel. Bookmark it or print it out!