The role of the Literary Agent is to negotiate a contract for your book with a publisher that you wouldn't necessarily have direct access to.
If you want to get with one of the big New Yorkpublishers, you will need an agent. However, be sure that any Literary Agent you submit your manuscript to is a member of the Association of Author's Representatives Member (AAR) or some other type of third party association governing the ethical practices of their member agents.
The internet or phone book are great places to find literary agents. But as with all other things, interview them about who they submit their manuscripts to - how often they've been picked up, their rates and get a review copy of their contract.
Ask for references!
Are they worth it? Ask the authors who have used them or tried to use them and after disseminating that information, make an informed decision.
Note: If you hire an agent, the contract that they negotiate with you states that they are the ones paid, then they take out what they are due, then in turn pay you.