Patient7: Anything you create is
automatically copyrighted! Ain't that cool? I've had stories published, and I never bothered to copyright them.
The reason to register a copyright is that you can't sue in federal court for a copyright violation unless you have the copyright registered. But, you can always register the work after you discover a violation.
I imagine a publishing company would handle copyright registration for you, but I've never had a book published, so I don't know.
If you want to register, you can
do it online for $35. You still have to mail the Library of Congress a copy of the work.
Your first step in contacting a publisher is to send a query letter.
Here's an article on how to write one. Be sure to read "The Writer's Guide" at your local library to find the addresses of publishers and get some tips on submitting.
Congratulations on nearly completing your book! Good luck on getting it published. You're my hero if you can get a book published while your still in your teens!
(I see Derf beat me to the punch, and gave better advice too!)