For you the answer is 'No'. For Stephen King, probably yes. I'm not being glib here. For fiction and screen plays, unless you're Stephen King or John Grisham, you must complete your works before querying an agent - especially as a newbie, an unknown commodity.
And then you don't hire an agent, you query them. It's like an audition, where in the span of a few paragraphs, you are trying to show why your idea is unique and marketable, while at the same time give the agent a flavor for your writing style and voice. Querying is an art form. If you manage to entice an agent, they will request more chapters and possibly the whole manuscript. They will tell you whether or not they are interested in taking you on.
You can learn a lot about query letters on agent blogs like Query Shark and Miss Snark. Learn how to query, before you try it - even have other writers review your query prior to sending it out. Look at the Preditors and Editors website (P&E) to find legitimate agents. There are a lot of scammers out there trying to get into your wallet. You NEVER pay an agent. Agents earn their wages from a percentage of what they get you from the sale of your work.
One other warning. After you find the legit agencies on P&E, go to their websites and research. Most have submission guidlines, and they vary from agency to agency. Some will want only a query letter, some will allow you to send manuscript pages as well, some work only via snail mail, some prefer e-mail. Follow the submission guidelines to the letter. Deviating can result in immediate rejection. Also, read the agent bios on most sights. Not all agents represent all genres. Find out which agents represent the genre you've written, and address your query to them; i.e., Dear Ms. Reid,.
Okay, now my fingers are as tired as your eyes, but this is fairly involved process.
Good luck, -Craven