Finding an agent/ publisher is the first step to selling your book proposal. However, even after you've sold your book proposal, you'll want to stay current with agent and publisher news to sell your next proposal, and the one after that.
Start a contact list of literary agents and publishers; as you find snippets of information online, or in your offline reading, enter notes into your database. Information you might want to add includes: recent sales and the amount the book was sold for, movements of editors from one publishing house to the next, and publishing house changes.
Collecting and maintaining all this information shouldn't be viewed as a chore. It's vital business intelligence. It could also help you to increase your income by many thousands of dollars each year.
Do you need a literary agent?
Yes. And no. It can sometimes be harder to get an agent than it is to get a publisher, so it's a good idea to query both. When you get an agent, you can tell the agent which publishers you've already queried. If you get an agent before you get a publisher, you can approach agents who are a good fit for your book to ask them if they will handle the contract negotiations for you.
You definitely need an agent if you intend to write more than one book. As to whether you should go agent-hunting, the answer is a definite yes. This isn't only because a literary agent will take a lot of the submission and negotiation workload, and because the agent has (one hopes) her fingers constantly on the pulse of publishing and knows what's going on, but it's also because an agent forms a handy cut-off switch between you and the publisher. When problems occur --- let's say that your editor's demands annoy you, or that your advance payments are late -- you have someone to gripe at other than your editor.
On the other hand, if you'd rather keep all the profits your book makes, and feel that you can handle your contract negotiations yourself, you may want to skip agents, and focus on publishers.
Many large publishers will not look at unagented material. However, this still leaves many who will. And most will look at any letter that you care to send them. Because a publisher can buy your book, and because you're likely to get a much faster response from a publisher than you will from an agent, I recommend that in addition to sending out your queries to agents, you also send them to publishers.
Once you start marketing your proposal, you'll find that some agents and publishers include words like "no multiple submissions" when they're telling authors how they want to receive proposals. In other words, they want exclusivity. Unfortunately, there's a big problem with this. The problem is time. Most agents and editors will take a month or longer to evaluate your proposal. Some take as long as six months. Considering that you may need to approach 20 to 30 editors and/ or publishers, you could still be sending out your book three years from now. Professional writers ignore these admonitions, because if they didn't, they wouldn't eat.