There's a huge amount of information here at AW on this topic and you'll get different advice from different people. It depends on what genre you're writing in and what sort of publisher you think is best to publish it.
Some publishers won't accept submissions except from agents. If this includes the publishers you think are the best fit for your book, then an agent would be the way to go.
Some publishers do accept unagented work. But these publishers get thousands of submissions. If you have an agent submitting for you, the publisher assumes the agent has already reviewed your ms. and wouldn't be submitting it unless it's decent. This gives you a step up above he other submissions.
Agents are in a position to negotiate better deals for you and to watch out for your rights when negotiating the contract with the publisher. Unless you are familiar with publishing law, publishing contracts, and what terms are being offered to other new authors in your genre, you won't have any idea if the offer a publisher makes you is a good deal or not.
Publishers frequently respond more quickly to agents than to individual authors. At least, that's what my agent tells me.
Finally, it's a huge psychological relief not to have the burden of submitting and receiving rejections. If you're doing it yourself, you're on a roller coaster of hope and despair as you send out queries, partials and fulls, and wait for responses. That's a lot of energy that you could be putting into writing.
On the other hand, the agent's going to get a percentage of your income.
If I were you, I'd take a look at books similar to your own to see who published them. Then find out the submission guidelines for those publishers. Also, check the acknowledgements in those books to see if the author thanks an agent--then you'll know if an agent was involved in the sale. Once you get a picture of your section of the market, you'll be better able to make a decision about an agent.
