It’s easier to gather information once you have a relatively narrow topic. A good analogy is when you conduct a search in an online database. You’ll get thousands (if not more) entries if you use the key words Vietnam War as opposed to fewer and more focused entries if you use terms related to economic impact of the war on the U.S.
Or, if you’re analyzing The Great Gatsby, you’ll be able to gather more specific information from the novel if you focus on a character, a theme, etc. instead of all elements of the novel at once.
It may help to use the image of a hand fan in order to understand gathering information. Think of your narrow topic as the end of the fan, the point at which all of the slats are linked together. As you gather information about your narrow topic, the fan spreads out, but the information is still all connected to the narrow topic.
You might use a general reference source, such as an encyclopedia, a textbook, a magazine, or a website to get a broad view of the issues related to a topic. This, in turn, helps you think of ways to narrow the topic in order to create a focused piece of writing.
However, it’s important to remember that sources like encyclopedias should be starting points only and should not be the kinds of sources you use in most college-level essays.