Don't! Not yet!
Okay, let's back up a step. If you've just completed National Novel Writing Month for the first time (http://nanowrimo.org) you're probably riding the crest of your success, thrilled with your book, and already thinking of what to wear at your first book signing.
I get it. Even though I'm a jaded old lady who knits socks on cold nights, I get it. Right now, you love your book. Love it with every last atom of your heart, and you want it in the hands of readers who will love it just as much as you do and sketch pictures of your favorite scenes to post on their tumblr pages where they tell all their friends to buy your book. You can't even think "my book" without thinking 💜my book💜. I've been there.
Do not try to publish it yet.
1) Your book needs time.
Let your book "rest" a bit so you have time to come down from the endorphin rush. You're in love. You're producing so much oxytocin that you could singlehandedly power a rocket to Neptune and then ride a gravity whip out to Pluto. That's not the time to make any kind of judgment about your book.
Go read someone else's book and force yourself not to look at your manuscript for a little while. Later you can come back to it and be a little more objective about the main character, the plot, the setting -- you know, the small details people tend to want to hang together in a story.
If you have a nagging concern in the back of your mind about one specific part of the story, it's probably correct. Even if you don't yet know how to fix it.
2) Your book needs feedback.
Find a few avid readers who aren't afraid of tears or screaming, and ask them if they'll read the first draft of your book. You might have to look online for what's known as a "beta-reader" but do find one, someone who will read through the story and be unafraid to voice all those repeating concerns you had in the back of your head. You know, about things like the main character, the plot, the setting... It has to be someone who's not afraid to say things like, "I didn't care what happened to your main character" and "Why didn't he just get in his car and leave the house full of spiders?"
(Yes, even if neither of those things apply to 💜your book.💜 A beta-reader must be tactful but fearless.)
It doesn't feel good to get negative comments, but trust me, when the book is published, no one will hesitate to bestow them upon you by the crate-load. And if you're going to publish traditionally, agents and editors also won't worry about your feelings. If you get a negative response at all, it will be along the lines of, "Not for me." You'll need actual feedback.
3) Your book needs detailed critique and an editor.
Once you've gotten through some beta readers, you want a critique partner to go over the book with you on a much more detailed level. Ideally this should be another writer, that way the two of you can chew on different solutions to complex problems. A beta reader might know the plot is confusing but a critique partner will be the one to point out that these three characters could be combined into one character without any damage to the plot, or that the main character's stakes should be raised in Act III (and then make a suggestion on how to do it.)
4) Your book needs a sharp query and an interesting synopsis
Get both of those ready before you start pitching to agents. Make sure you know exactly how querying works and what to expect when you approach agents. Learn what agents do. If you're going to approach small publishers, you'll need to know what they do too, and the kinds of things they want to see. Get other eyes on your query to make sure it's a tantalizing sales letter for your book.
5) Your book needs not to fall prey to scammers.
The larger NaNoWriMo gets, the more predators are going to try making money off it. Before you even consider publishing your book, you need to learn how publishing works, both traditional and independent. The QueryTracker blog is a good start (this site, if you got here by googling "publishing my nanowrimo novel") but you should also find guidance in writing groups. Double check that any service you use is not a scam.
Traditional agents do not charge money to read or represent your manuscript. Traditional publishers do not charge money to publish your manuscript. You should not be bound by contract to purchase a certain number of your own books. You should not have to earn back the publisher's net expenses before receiving royalties. You should be the owner of your own copyright. You should not be forced to sign a non-compete clause. You should have a lawyer review any contract you sign and be prepared that every sentence of any contract will be leveled against you in the worst possible way. If you can't abide by the strictest interpretation of the document, don't sign the contract.
Many writers are desperate to get their stories out there, but if you try too soon, you will undercut your book's success. Your story...I mean, 💜your story💜 deserves the best you can give it. That means time, editing, and honest business practices.
You finished! Congratulations! Now give 💜your book💜 a huggle and tuck it in to rest for a little while.