What topic in Cocktailparty could you write an Essay About?

There's an excellent summary of the play at the website linked below. You might want to look into the character of Celia, as a morality play, Eliot seems to use her character as a martyr for Christian ideals. This play is all about choices; the first paragraph of the article;

"The Cocktail Party is a twentieth century morality play, one that argues that people must accept their burdensome roles as decision makers. Unlike medieval morality plays, which upbraided audiences to choose good deeds and shun evil, The Cocktail Party simply places its characters in the crucible of choice, and diagnoses their indecision as a malaise. Eliot means to demonstrate that it is the burden of the human race to face tough choices, and live with the consequences."

I believe bookrags has a complete study guide, but it does have to be purchased.