First you should send it to an agent, and then they'll present it to publishers.
There's a lotta stuff to consider when sending your novel to an agent. First and foremost: are they legit? Then there's how much of your novel to send them (usually its the first three chapters at first), and then how to format the manuscript, a whole bunch of similiar stuff.
If you're interested in trying to publish a novel, buy the Writers' and Artists' Yearbook (or something similiar). That'll have all the information you'll need, plus a big ol' list of legit agents and publishers.
I posted a thread a while back that is stickied at the top of this forum... "Essential websites for writers trying to get published," or something like that. Those sites gave me a lot of information, and i'm sure they'll help you.
“We’re still here,” he says, his voice cold, his hands shaking. “We know how to be invisible, how to play dead. But at the end of the day, we are still here.” ~Dax
Teacher: "What do we do with adjectives in Spanish?"
S: "We eat them!"
The first step in trying to get published is editing. Honestly. Spend as much time as possible making your novel as good as it possibly can be. You don't want to send a half-worked manuscript to an agent.
Also, Sureal said to said the first few chapters, which isn't a bad idea, but I read somewhere that you shouldn't send anything unless they request it. It also said that if you're querying by email, you could attach a few chapters. So long as what you're sending doesn't take up space on their already cluttered desk, you should be good.
Research the agent in full, not only to be sure they are legitimate, but to be sure they are the right agent for you. If you write science fiction stories about monkeys taking over the world, you probably don't want to send your manuscript to an agent who helps get cook books published. Try to find an agent who fits your genre, and knows the kind of work your doing.
Also, don't just send to one agent at a time, send to multiple. It can take a long time to get a reply, and you don't want to be waiting forever. It's OK to deny nine, and accept the tenth, and best agent for you.
Then, from there, the agent helps you send it to a publisher. I hope that helped! Just keep in mind that, for certain, editing is the key. I'm going to spend up to the next four years editing my novel.
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The first thing you'll need for this is a Query letter. However, before you construct the query letter, I completely agree with Suzanna that you should edit edit edit. The biggest mistake of aspiring authors is to send out their work too soon. Don't depend on the agent or editor to clean up your work. That's not their job. Your work should be the best it should be when sending it out so it has the best possible chances of getting an agent and therefore getting published.
Okay, the the query letter. You'll need this regardless of whether you're going through an agent or directly to publishers. A query letter is a short letter to the prospective agent or publisher proposing your book, listing your credits (if you have any) and why you were the right person to write the book, and why your book is that much better for it. And it really shouldn't be over a page. This, in my opinion, is the most difficult part of the writing process. As far as emails go, agents really don't like attachments unless they've requested it. They won't even open them, with all the viruses going around nowadays. In any case, I believe it's always better to query via snail mail. Most agents prefer it, and I find that the delete key is much too tempting for an agent when dealing with email. If there's a physical piece of paper, I think it has more of a chance of being seriously considered. That's just my opinion.
Secondly, you need to consider whether or not to go for an agent. Most published authors do, and if you want to get publsihed by any of the big-name publishers (Penguin, Random House, etc.) you really have to. However, if you're targeting smaller, local printers and publishers, than you might get away with going without an agent. That's entirely up to you. Keep in mind, however, that when going with smaller presses, the runns are going to be smaller, and many times the book won't be found in large bookstores such as Barnes and Noble. In fact, the book might pirmarily be available online. Also, you should really research each and every agent you query. You want to make sure the agent you're sending to represents your work. Don't send a Sci-Fi novel to an agent that primarily represents Romance. That not only wastes both the agent's and your time, it also leave a bad impression on the agent if you ever want to query them for another purpose, and will earn you a quick rejection. You can also look at past books an agent has represented and see if they're similar to what you're proposing. This is always good to put in your query letter.
After you send out a round of queries (yes, a round. As mentioned, not to just one agent ), you wait. Once the responses start rolling in (and be prepared, you WILL get some rejections...it's a fact of life), you take it from there. The big thing is to send an agent exactly what they want, no more, no less. In addition to promoting your writing, you're also trying to prove you'll be easy to work with in a professional relationship. If you're constantly sending an agent more than he's requested, it'll send the message that you might always be pushing more books on him to represent.
The publishing process is a very slow and arduous process, and one I've studied extensively. If you're like more information, or what I can give you, anyway, , or would like me to explain this process in a little more depth, feel free to PM me.
Oh, and a really good source (other than the Writer's Market, of course) is: www.agentquery.com
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There’s always a story. It’s all stories, really. The sun coming up every day is a story. Everything’s got a story in it. Change the story, change the world. — Terry Pratchett