Q - At what point in a writer's career does he/she need an agent? What is the best way to obtain an agent? Can I submit to more than one simultaneously?
A - Because there are so many publishers listed in the market guide that do not require an agent, there is never a point at which you must have an agent. However, if you want one, then you need to wait until you have a finished manuscript before approaching them. If you decide to look for one, be sure you do that before you do any submitting. An agent does not want to take on a book that has already made the rounds.
There are a number of ways to find an agent. Attend conferences where agents are in attendance and taking appointments. Ask writing friends to recommend their agent, or better yet, recommend you to their agent. Some agents are only open to new clients through these two options. If these are not possible for you, you can write to any agents listed in the market guide that indicate they are open to new clients. And, Yes, you can send out simultaneous queries (unless their listing indicates they don't accept them). The listing will usually indicate what types of material they handle, so look for those that handle what you have to offer. Also be sure to query them in the manner they suggest. Many will accept an e-mail query, where others want only a written query or proposal by mail.