First of all, mazel tov!
Your next step is to publish the book. This means that the book is in a final form and either printed on paper with a cover or in an e-book format. It also means you have an ISBN number and all the other technical bits that go along with publishing.
You can start by sending out queries to agents or publishers. I recommend using the Children's Writer's & Illustrator's Market 2019 as a source of information about both agents and publishers. Each will have different requirements for submissions. Many publishers only accept submissions from agents. Some don't accept submissions at all. Others will want books targeted for particular audiences or on certain topics.
Follow all instructions to the letter! The best way to get your query deleted is to ignore their rules.
Also go to conferences, attend conventions, and join your local and national professional writing societies that are relevant to you.
If you can't get an agent, go for the medium to small publishers that accept direct submissions. If the publisher charges you money, run. That's not a publisher, it's a vanity press. You can also start with a small publisher. There are likely some local to you. You don't need to do local, but it's nice.
If none of that works, or if you just prefer this method, go for self-publishing. I recommend finding a company that does the tech setup for you. It shouldn't cost more than $200 and you'll get your ISBN number, formatting for paper and e-books, and the setup for each. Of course they'd charge more for the actual book copies. Make sure you have a contract where you own your work 100% and they don't get any money besides what you give them up front for the service.
If you self-publish, pay for an independent editor and proofreader. Don't do this yourself! This can be two people or the same one, but they are different skills and tasks.
To sell a book, you must first publish it. No bookstore will accept a book that doesn't physically exist. They won't publish it for you either. Many bookstores do accept self-published books (ISBNs are mandatory); the fabulous small chain local to me allows a certain number of self-published books by local authors at a time. There's a waiting list.
If you self-publish, you can sell at Amazon (e-book and paper, though the latter is a bit harder). If you do traditional publishing, your publisher will submit to Amazon. Use paper books to donate to your local library (yep, they'll usually accept them) and for tabling at events. Or just to sell to friends. E-books will have a wider audience.
Marketing is a whole other issue and way out of the scope of this question. But it's important.
Each of these things I've mentioned could be their own question. And probably already is! So check the archives. There are plenty of other things I didn't mention at all. I mean, there are entire books answering this question (again, see Writer's Digest). If you have more questions that aren't duplicates, go ahead and ask.
