That is indeed the question.
When it comes to NaNoWriMo, there are basically two approaches you can take in planning your novel. They are as follows:
Option #1: Pantsing. Derived from the expression “flying by the seat of your pants,” this approach involves little to no planning whatsoever. Basically, when November comes, you just open a new document and start typing away, making everything up as you go. I’ve never “truly” pantsed a novel — my second novel had the least planning of any of them, but I still basically knew who my characters were, I knew where the plot would (very) generally go, and I had a few ideas for some specific scenes. I would personally not recommend pantsing your first novel, or even your second or third, unless you’re crazily spontaneous when it comes to creativity and have zero worry about producing a bad first draft. (You shouldn’t be worried about producing a bad first draft no matter what your approach is, but still.)
Option #2: Planning. There’s a pretty wide spectrum of planning: everything from just your main character and a general outline all the way up to a detailed roster of all of your characters, a description of what happens in each chapter, a written-out character arc for each character, et cetera. I’d recommend this approach, especially if you’re a newbie, but beware of over-planning. Preparing too much of your novel beforehand can actually hamper your creativity, not to mention productivity, when you actually sit down to write the thing. At the very least, you should know your main character(s) and have a basic idea of the overall plot.
Whichever approach you choose, be sure to have fun doing it! NaNo is less than two weeks away.
Which reminds me… I’d better get planning. Meep. o.o