Traditionally, NaNoWriMo works best when you start a brand-new project. It may be an arbitrary distinction, but we’ve seen that novelists do better (and have more fun) when they’re free from the constraints of existing manuscripts. Give yourself the gift of a clean slate!
That said, we welcome all writers at any stage. Outlines, character sketches, and other planning steps are encouraged, and you're welcome to continue an old project. Just be sure to only count words written during November toward your goal.