If your goal is to take the traditional publishing route then yes, you should be seeking an agent. Let me do clarify one thing though….if you are submitting to indie publishing houses then no, most will accept manuscripts straight from the author however if you are looking at large conglomerates then yes, you need to find an agent. Most of these will only accept submissions from agents.
Word to the wise…there is no formal education that a person must obtain in order to call themselves an agent. In other words, just ensure you use caution and do some serious research before reaching out. You want to find someone that not only knows the industry inside and out, but someone who has some serious ties to the publishing industry.
Here are some words of advice passed along from Ginny Wiehardt (http://fictionwriting.about.com/od/thebusinessofwriting/tp/agenthowto.htm):
1. Finish Your Novel.
Finish writing your novel before contacting literary agents. Have other writers read your book, take a novel writing class, or hire an editor. We all have blind spots as writers; identify them before contacting any agents with your novel. And make sure that the first 30 pages or so are especially strong. Agents need to see that you can set up your story effectively.
2. Establish Credibility.
An agent will be more likely to pay attention to your work if you have published in smaller journals and magazines. Find an excerpt from your book that can stand alone and begin sending it out. (Other ways of establishing yourself as a serious writer include winning a contest or getting an MFA.)
3. Research Literary Agents.
In finding an agent, you’ll do much of the same research you do when looking for a job. And as with a job hunt, there are many tools at your disposal. Learn about the essential ones here.
4. Write a Query Letter.
Your query letter is your one-page audition: your chance to catch an agent’s eye. Introduce your novel with a short, compelling synopsis, then share the credentials you’ve been working so hard to accumulate. These dos and don’ts will further help you craft a professional, dynamic letter.
5. Start Writing Literary Agents.
Once you’ve finished your novel and researched the market, you’re ready to start querying literary agents. Dive in and see what happens. Even if you don’t land an agent right away, hopefully you’ll get feedback, both about your writing and about where it might belong.
As you proceed, questions will inevitably come up: What fees should you expect? Can you send to more than one at a time? What if you have more than one book? For answers, refer to the agent FAQs. You can also follow one writer’s path to finding an agent.
You CAN do this! (Evin – http://www.mrgurupublishing.com)