The second sentence in the introduction usually develops the first (general) statement. So it may be a similar topic as the first sentence, but more specific. For example, with the essay question: "Smoking should be banned. Discuss both sides of the argument." Your first (general) statement may be, "There are many factors in the world that can affect our health." The second sentence may develop the idea of health but make it more specific by talking about 'smoking'. For example, "One of the most commonly-mentioned factors is smoking, which has been researched globally."