It is preferable that the topic be one you have dealt with in at least one of your two previous portfolio papers. The essay must have an argument, something that you will attempt to persuade the reader of, stated clearly in the introduction of your essay. The argument is also called the thesis. Topics can usually be expressed as nouns or noun phrases. Not all theses are good ones.
People have a marked tendency to make theses which are too broad.
The essay should have an introduction , a body, and a conclusion which recaps the argument but does not repeat it verbatim. Although I'm asking you to clarify your main idea in the beginning, you should be aware that not all essays follow this format. If you wish to experiment with an unusual organization, write an explanation of what you have done and why in your cover letter. You may also use personal experience, logical argument, personal interviews, or Internet sources when appropriate.