If you're reading this post, I find it safe to assume you feel you are ready to begin the publishing process and are looking for a direction to go in. Great. But before you dive in, there are some very important things you need to make sure you've covered.
You may feel your masterpiece is finally complete, but is it really? Are you sure it's the absolute best it can be? Have you had any professionals (editors, agents, published authors) review it? If so, have they fueled your fire to drive onward? Have you completed the perfect synopsis? Jane Friedman wrote a great post concerning these questions. She asks three wonderful questions that you should read and pay close attention to. Visit 3 Questions Every Creative Person Must Ask.
If you've answered yes to these questions then you are probably ready to dig in. Now, get an agent.
The publishing business can be a tricky world to play in. You have to be up on the rules or you could lose the game. For example, when querying agents you need to understand that not all agents are alike. They have different tastes, feelings, and guidelines that you must understand before submitting a query. Check out Agent Query for information on writing a great query letter. Make sure you know the agent you are submitting to. I can't say this enough - make sure you know the agent. If you don't follow the agent's guidelines exactly you could find your great work of art at the bottom of a slush pile without so much as a read of the second sentence. Know your agent.
Put together the best you can. This is your foot in the door with agents, your "one shot deal" so to speak. Make it good.