There's a great deal of contradictory advice about whether you need an agent or not. Some people will tell you that most publishers will consign any unsolicited manuscript, other than from a reputable agent, directly to the 'slush pile'. (That seems to be an exaggeration since no publisher would want to turn down a masterpiece without at least looking at it!). Others will tell you that the best agents only handle recognised writers, so those which will represent new writers can't do much to help them. (There might be some truth in that statement but all agents like to make money and they can't do that unless they can find good manuscripts and promote them well).
In practice it's probably best to produce dozens of copies of your manuscript in standard publishers' format. (That's double-spaced, loose leaf etc. The W & A Yearbook should explain it all). Then bombard every possible publisher and agent with your work. (The information in the Yearbook will help ensure that you don't waste your time and money by, say, sending your fictional work to a publisher who only handles academic non-fiction. However it's also worth looking in bookshops to see who publishes the type of work you've written, so that you can target your submissions accurately). That 'blitz' technique is exactly what J K Rowling used when she'd written her first book, so it's certainly worth a try!
Chris