In principle, I agree with KTB and erech, but unfortunately, due to the web and all the self-/vanity-publishing services, the market is unimaginably flooded with would-be Neresians (or Eggerses, or Cory Doctorows, or that guy who wrote
Bomb The Suburbs, or that other guy who wrote
The Tawacores). This is not to discourage you, because you're surely brilliant enough to be the exception to the rule, but generally speaking, succeeding this way will take a massive about of energy, self-promotion, and luck. It also makes sense to start a journal with your buddies, like McSweeney's, n+1, or Yeti, and get one or two pieces by famous writers in there to garner attention for everything else in it, if you're up for that. You're probably better off submitting to a small press, though, if what you're doing is more unconventional or whatever.
I'm assuming this is fiction, as nonfiction follows different rules, but correct me if I'm wrong.
Martin is right - it's important to have it finished before you submit anything anywhere, because you need to have a really specific idea of what it is in order to just describe it accurately. People that work at agents and publishing houses read zillions of these things and can tell whether or not the submitter knows what s/he is submitting. So, finish it, and make sure it's finished (at least one rewrite) and once you're finished, double-space it, and come up with a pithy, super-specific, and professional-sounding one-page synopsis that will fit on a query letter. Then either pay for or pirate a subscription to the Writers' Digest website and send this letter and a few sample chapters (20 pages max) to agents and small presses. Some other tips:
-Use whatever personal favors you can cash in -- seriously. If there's a chance you can honestly say "so and so said you might be interested in my work," do it. Ask any friends you have in publishing or even related industries to read it.
-If you submit directly to larger publishing houses, your work will go to the slushpile, but it will definitely be read, probably by a 20-something assistant. If s/he likes it, s/he will pass it on. But even in the best-case scenarios, unagented writers who get in this way often get screwed out of subsidiary rights and so on.
-If you submit to an agent (the standard way to go), be as specific as possible -- use any personal connections you have, and barring that, know who they represent and say why that means they should add you to their roster. If one agent bites (and they're legit and you want to work with them), you should email the others as a courtesy (and/or to initiate a possible feeding frenzy, though that's
unlikely). Agents have guidelines on their websites and you should know and follow them.
-If you approach a small press directly, you're more likely to be treated like a human being, though there's little to no money in it (though corporate publishing is collapsing anyway and these guys are probably the future). Again, all of the above follows, but be as specific as possible and try to show that you give a shit about what they publish. So, if you like the Alex Cox memoir published by Soft Skull, query them and mention briefly that you enjoyed that book and
Repo Man changed your life, or whatever.
I learned all of this by standing behind John Hodgman in the phone room last week.
