There are a number ideas, themes, and essay topics to pull from this novel. Dignity is treated in an interesting way in the book, both topically and thematically, and it can be argued to be central to Jake's character.
He admires the bull fighters and the sport of bull fighting, in part, because of the dignity he finds in the sport. He also holds himself back often, for dignity perhaps, and later completely loses face, getting himself arrested.
Many of Jake's decisions and conflicts can be illuminated in the light of dignity as a theme.
A prevalent trope of Hemingway is finding meaning in life. In The Sun Also Rises, Jake remarks, "Nobody ever lives their life all the way up except bullfighters." Cohn replies to this, "I'm not interested in bullfighters. That's an abnormal life." However, the group rejects Cohn's romantic vision of having mistresses and literary fame and adventure. Finally, it comes down to the key of life being the ability of a person to wisely use one's money. While Brett and Mike Campbell are incapable of doing this, Bill relies upon exchange value and use, purchasing moments and using them to display one's stature. Jake, on the other hand, uses his money for tabulating and distracting himself from his impotence. At one point in the novel, he utters a very telling statement:
I paid my way into things so that I had a good time. Either you paid by learning about them or by experiences, or by taking chances, or by money. Enjoying living was learning to get your money's worth and knowing when you had it. You could get your money's worth. The world was a good place to buy in.
This "lost generation" of which Hemingway was a part himself, used money to find enjoyment in life until a semblance of reality was created and the war ended, until "the sun also rises."
See the enotes on themes and characters. Usually an analysis of theme of one of the main characters provides interesting topics.
You haven't said what kind of essay you need to write. I've pasted below links to a few sites that will help you. The first link is to the article How to Write a Good Essay on English Literature. This article gives five steps to guide you in writing your essay. Even though the article is about English literature, if you omit step 3, the suggestions work for just about anything. The second site is an article on how to write introductions. The last link is to an article about how to write a summary. If your assignment is like a book report, then you'll find the information in this article very helpful.
Probably the most important advice these sites give is to focus your essay. Find some aspect of the novel that is particularly interesting to you and use that as your thesis. The best essay will come from what you like or dislike about the story. Read the eNotes introduction, overview, and analysis of themes for extra help, but let the topic come from you.