What should I put in a good thesis?

For an essay on a piece of literature, a good approach is to choose a unifying main theme to discuss, and to do so via a three-pronged thesis. The three prongs of the thesis will ideally each get one paragraph's worth of discussion, to be wrapped up in a conclusion. These three prongs should somehow express the main theme chosen, e.g. literary devices, specific characters, or historic background. An example of a good thesis might be, "In Othello, Shakespeare develops the theme of good vs evil through the relationship between Othello and Iago, through the symbolism of color, and through tone." Then each of the three prongs gets one paragraph explaining, with supporting evidence, how e.g. the tone is used to develop the theme of good vs evil.