THese days there are a lot of self publishing (and promotion) options, you might have to get more savy about social marketing on the internet, creating an author site, etc.
There are people making a decent income self publishing Harry Potter like serial books, here is some self publishing info:
amarketingexpert.com
writersinthesky.com
inktreemarketing.com
ebookfriends.com
worthyofpublishing.com
Some other writer resources
Teenink
Quizilla
Inkpop
Storywrite
Booksie
Wattpad
Fictionpress
Mibba - young writes share.
Writerscafe.org pier reviews of poetry, short stories, novels, scripts, and screenplays
Youngwriterssociety
Writers-Network.com
There are also print on demand services that will print and direct ship book orders one at a time, so you don't have to have a garage full of pre-printed books as you once had to.
This example also handle drop shipping to USA (overseas orders cost considerably more)
http://www.Lulu.com/
(I understand you have to pay about $200 for an ISBN# to have a physical book sold on Amazon)
Most real (not vanity press) publishing houses won't accept unsolicited submissions from adults. What you need to do is finish the book and then submit to a literary agent for review. If the agent believes it is salable, he will sign on as your agent and try to use his/her contacts to get you published.
You asked question in wrong category and that's why there's little chance to get appropriate answer.
I'm actually not a teen yet (I know, a bit young to be writing a book) and I am in the process of writing a fantasy novel which I plan (if it gets published) to make into a tetralogy. By the way, don't underestimate me because I'm young. I'm a pretty good writer. Does anyone know a publishing company that will accept unsolicited work from teens? Thanks for any help.