In Fitzgerald’s classic novel The Great Gatsby, the theme that resounds throughout the novel is the corruption of the American dream.

Possible continuation in the opening paragraph: In this essay I will show how four major characters, Daisy, Gatsby, George, and Tom, exemplify this corruption.

Cheers,

Bruce

First, Gatsby demonstrates the failure of the American Dream in his [whatever his failure is]…

Next, Daisy demonstrates the failure of the American dream by her [whatever her failure is]…

Thirdly, George’s failure demonstrates the failure of the American dream in his [insert failure here]…

Finally, Tom demonstrates the failure of the American dream by his [insert failure here].

Of course you can jazz up the writing with nicer verbs and whatever, but that is basically all you have to do for a topic sentence: summarize what you’re about to tell them.