Okay, so the first thing you need to do when you decide you want to publish your book or novel is get it copyrighted (I recommend you do this even if you don't want to get it published that way no one can steal it and try to publish it themselves and you don't get credit for any of it because it wasn't protected).
Once you do this all you have to do it wait.
Unless if you want to start writing your query letter. If you have already read about what a query letter is you can skip the next paragraph.
A query letter is basically a book proposal. It has to tell about your book and (if they request it) about yourself. The link below will take you to a website, scroll down to the bottom and click Winning Query Samples (bottom of the webpage, top, left).
Look through all of them. You can read through these to figure out a way to start your query letter. Don't get discuraged writing your letter. I rewrote mine at least a dozen times until I felt like it was good. The trick is not to give too much away, but you also want them to catch a little bit of the story. Make sure your spelling and grammer is correct too, they pay attention to that.
Here, I'll also give you the layout of my query letter:
I am currently seeking representation for my complete (type in how many words long your book is) young adult novel (example, you can put the genres of your book), (title of your book). I am a big fan of the YA authors you represent. (Most of the time I've never read the novels they represent, but I put it there anyway).
(Then you put a short overview that will hook your agent).
Upon your request I will send you my complete manuscript (NEVER send a full manuscript to an agent or publisher unless they ask for it). Thank you for your consideration.
Then you put your name, Email address, and your phone number.
Back to me
That was the query letter I wrote. I just took out my overview, word count, and title. My book it copyrighted, but I don't want anyone stealing pieces of it.