Here are some things I picked up from reading a book on writing by Orson Scott Card:
1) Just send in the first chapter to the publisher and let him know how much you've got so far. They should be able to judge the work's worth without you sending, or them having to deal with, a huge block of paper. They will contact you if they need further chapters to make up their mind.
2) Send your submission to one publisher at a time - if they refuse it you can send it to the next, but if 2 publishers say they're interested at the same time you just put everyone in an awkward position. (I know this will take ALOT more time, but you're establishing a reputation as a writer - don't blow it the first time out).
3) If you get an acceptance offer from the publisher then it's time to get an agent. An agent will be easier to sign on once they know you've got a winner (and hense, their fee). Agents don't typically put out much effort for a new, unproven writer.
4) Getting a publisher to read your work is probably the hardest part. Here's some tips to help in that regard:
a) Join a creative writing club or take a class - this will get you
started on networking in the industry. Not only can
published writer's use their influence with a publisher, but
publishers themselves look in these institutions for raw
talent. You'll also have a pool of other aspiring writers to help
critique and help with any awkward places in your work.
b) Get an established writer in your genre to read your book
before the publisher. Some writers will be helpful, others
may even charge you. But a recommendation from an
established talent will get your foot in the door.
5) Lastly, check out a book on publishing in your genre. The
list of publishers changes constantly and, to a lesser extent,
their criteria for acceptance. There's some book out there
called "Writer's Almanac" (or something) that's published
every year with all sorts of helpful hints.
Good luck, dude. Don't worry about still being in high school, just
don't tell anyone till it's time to sign the contract and your parents
have to cosign.
- sudz