First thing you want to do is find a neutral beta reader. These are people who don't know you at all and will give you honest feedback. Teachers and friends might be great for cleaning up grammar, but they're not going to tell you if it's absolute crap. I've had some good experience with the ones at Perfect Imagination
The next step is to research publishers. Make sure you pay attention to their submission details and make sure they publish the type of novel you have. You don't want to send a romance novel to a mystery publisher.
The next step is to write out a query letter. Most publishers will have guidelines on what they want from you in this letter. Common elements include a full synopsis (including the ending), any writing credentials that you may have, and why you want to publish with them. Some will ask for you to send a sample (usually the first couple chapters and the last) others will tell you not to and if they're interested in it, they'll request it.
Remember to proofread this letter and have someone else look over it. Always be professional and follow a publisher's guidelines to the letter. Keep it in your mind that editors often can work at multiple houses and that editors talk to each other. Trust me, we like to gossip about people who give us problems. Mouthing off or acting like an amateur with one editor, could cost you at a dozen houses.
After your query letters are off, pray to the Publishing Gods that someone accepts it. After you get contract offers, then you can seek out an agent or a lawyer to read over it and help negotiate to get you the best contract possible.
When you're researching publishers and agents, make sure you check them out. Look at sites like Predictors and Editors, ask around in publishing circles, or at the very least Google the publisher's name and "complaint".