Essentially, Othello can be defined by look at Iago as his foil. Othello is very idealistic and therefore sees things in a very ideal manner. Pay attention to how singularly focused Othello becomes after Iago convinces him that Desdemona is sleeping with Cassio. Iago is able to project his distorted view of the world in Othello's mind adn cause Othello to act in a tragic manner. If you wish to write an essay on Othello, you might want to chart his change after scene three.

You would start such an essay from two directions. First, you'd figure out what you think of Othello. Who is he, essentially? What are his defining characteristics? The section of the enotes study guide analyzing him would help with that: http://www.enotes.com/othello/36679

Second, you'd approach the assignment from the essay side. What will you prove, show, or argue about Othello? Focus on developing that answer into a good clear thesis and you're on your way.