I first got an agent, and she submitted to publishers on my behalf. It took ten years and two novels to get an agent (lots of rejection), but I finally broke through. The agent is the gatekeeper for the publishers, and can connect you to editors suited to or hungry for the material you write.
My advice would be to get your manuscript in as perfect shape as possible, and query agents listed in the acknowledgments sections of books similar to yours. Mention why the agent would make a good fit with you in your query letter.
I also found that attending craft workshops and using a freelance editor before submitting to an agent with the second book helped quite a bit.