You could look at this question in two ways.
First, the most important element in any writing - by far the most important - is content. The best writing is writing that has something important to say. This is in the form of a thesis. If there is no point, then there is little point is writing or reading, for that matters. So, ask yourself, if you have something insightful to say.
Second, form matters as well. In fact, in some ways, form also is a conveyor of meaning. So, it is important to keep matters of style in focus as well. This takes time and practice.
In short, content and form.
This is an excellent question for the reason that the answer is complex.
The essay as a form of writing is actually not well-defined. Of course, in K-12 education, we are taught to write the five-paragraph essay (introduction, three body paragraphs, and conclusion). However, the structure of an essay in its more general form is not easy to pinpoint.
The first step, I think, is to determine the audience for this essay.
Does your teacher or professor want a specific format? If he or she has already provided guidelines, then follow the guidelines.
The second step is to determine the purpose for the essay. Are you writing about a person? I can suggest three ways you can approach this (and, of course, there can be more).
First, you could write a chronological account of the person's life, much like a biography. Along the way, you include important or required information.
Second, you could write an essay about a person using themes that appear in that person's life. For example, did this person experience a lot of change? Success? Failure? Writing this type of essay may require you to jump around in time, but as long as you write carefully, the time should be clear to the reader.
Third, you could write an essay about a person using their personal characteristics. For example, you could discuss how a person's honesty has influenced events in their life.