How do I get my first novel published when I’m a kid?

Let me offer up some tough love here, if I may: it’s hard to get published by a traditional publishing house. When you go to the bookstore and see books from Wiley, Doubleday, Macmillan, Dearborn, etc etc., you have to realize that for each book that’s on the shelf, the publisher saw at least fifty proposals that were rejected, and for each book on the shelf, there were probably two that never quite made it to publication for one reason or another.
Add to that the fact that more books are published each year than at any time in the history of mankind. In other words, there’ll be more books published in 2008 than were published in 2007, which was itself more than 2000, etc etc.
To say that it’s a big, competitive pool is an understatement!
An important second factor here is that you’re talking about fiction. Even more than non-fiction / technical, fiction is an astonishingly competitive space. I mean, everyone who can tell a story, everyone who can write a letter or long email, every blogger, every Tom, Dick and Harry, has “that big novel” inside them and is convinced that they could be the next Tom Clancy or Elizabeth Diamant if they just get in the door. Ugh!
The third facet you bring to this is one that’s both a positive and a negative: your youth. There are certainly examples of very good, popular books written by teenagers (Christopher Paolini’s entertaining book Eragorn comes to mind) , but they are exceedingly rare, I’m sorry to report.
Should this all discourage you from proceeding with your dreams? Absolutely not!
Instead, I think you’ll be more likely to succeed if you go into this with a pragmatic, realistic perspective on the business and a realization that publishing is a business and that your book could be wonderful, but if they don’t think it’ll sell they won’t publish it.
If you’re serious about this, yes, I’d engage a book agent. Write a smart, thoughtful cover letter explaining your situation, include a 10-15 page sample from your best book, and send it off to every agent you can unearth who specializes in your genre. Expect a lot of rejections (I’ve been rejected by plenty of agents myself, even after publishing twenty books) but remember, you only need one to have a representative who can sell your work and help you succeed!
Finally, don’t forget that there are many alternative publishing avenues available nowadays, from blogging your book, section by section, to print on demand, vanity press, and much more. If you really want the credibility of a major publisher and a fancy book in the bookstore, I’d find an agent. If you just want to be published, starttalking with authors online and see what you can find out about how they’ve published their material.
Good luck to you! I look forward to reading your books to my kids in a few years!!