How do you decide what your essay is going to be about? Sometimes that seems obvious. You are writing because you have a specific message you want to communicate. You are upset about or enthusiastic over something; you want to convince your city to repeal a restrictive postering ordinance, or celebrate the beauties of spoken word poetry. The more specific you make your theme, the more specific an impression you will leave in the minds of your readers. If your theme is "the postering ban," all they may leave with is the memory, "a postering ban exists." An article is meant to be a balanced presentation of facts: what is a postering ban, why do supporters defend what it does, what are the arguments against it? An essay is a field for personal feeling and opinion. Why are you against the postering ban? Which reason do you feel strongest about? What you feel most strongly about you will write most strongly about. One way to narrow down a theme is to write a declarative sentence; then ask, "Why?" Your answer becomes another declarative sentence. You ask, "Why?" Repeat until the only answer you can come up with is "Because!" You now have an end goal, a fundamental concept on which to build.
Identifying a Theme
What was that about? A similar method can be used to identify the theme of an essay you read. State what you think the point of an essay was, in one sentence. Then ask, "What does that imply?" Make that answer a new sentence, and ask, "What does that imply?" This may help uncover implications in the essay that slipped by you at first. Maybe some implications that slipped by the essayist.