Editing your work, of course. Okay, maybe that's your second thought after the relief and excitement about finishing. It's still an important thought.
Where do you begin?
First, give yourself at least a week's break. I know, this sounds anti-productive but fresh eyes make all the difference! Work on another project or plot your next piece. Just keep your mind off what you just finished.
It's been a week. Now what?
Now you do a quick reread, highlighting areas that may need work. I like to use different colors to keep every idea separate- one for grammar, one for flow, and one for general areas I'm not sure about. I try to look at the manuscript as if I'm being my own critique partner, who is only looking for areas that may need work without necessarily trying to solve the problems entirely.
Of course the next step is to actually solve any pesky problems that you may have discovered. You can do the easy ones first - grammar, and words that disrupt the flow. All that highlighting you did earlier means that now you know exactly which areas are the weakest, letting you build them up in other scenes if needed. Maybe you highlighted that steamy kissy scene because it seemed too much like insta-love during your first read. Now you know to add in some small moments where your characters seem to be falling for each other, and are perhaps confused by their feelings.
Do you have critique partners? Now's the time to use them. Before I send everything to my critique partners, I like to do one more round of editing through reading each scene out loud. Hearing the manuscript out loud always makes a big difference for me to catch flow issues that I might not have noticed before. You know the type - sentences that sound great in your head but are clunky and unwieldy when someone has to actually say that.
Happy editing! As it's been proven, some great YA novels have derived from Nanowrimo - maybe your book is the next!