What interests you about the assigned topic? What do you know something about? These two questions should figure prominently whenever you write. Writing about something you know and like will not only produce a better, more informed paper, but will also make the writing process less laborious. You can cater your paper to fit any essay assignment (topic) and should want to write on a particular topic, not select one because your professor might like it. Yet, often in English Composition classes, you must choose a subject that fits a topic assigned by your professor (See How does a “topic” differ from a “subject”?).
The next step is to figure out if you can develop an essay that fully addresses your topic and the assignment in an orderly, appropriate essay. Like deciding on a subject, appropriateness depends on whether or not you can adequately cover the subject in the assigned length of the paper or time in which you have to compose it. A second consideration should be topic and whether or not the subject falls within its purview.
Use your common sense in selecting your subject. While writing on a controversial subject like pornography would be appropriate, discussing sexual relations in an essay would not. Finally, consider audience when deciding on a topic. Who is reading your paper? Your topic should appeal to the reader. Ignoring your reader’s interests or not considering who your reader is exactly will result in a less well-received essay.
Only write on that which you know.