How would you compare and contrast the Curtis brothers (Darry, Soda and Ponyboy), and the role each of them plays in Ponyboy's life?
Well done for recognizing the importance of structure in presenting a well-written essay. A good structure ensures that your essay flows logically and that each point you make is relevant to your purpose. In other words, structure keeps your essay focused on its topic.
In terms of The Outsiders and your topic, you need a thesis sentence in your introductory paragraph so think about the main purpose of your essay- the role of each of the Curtis brothers in Ponyboy's life. Therefore, you could consider an opening sentence that introduces Ponyboy as the narrator and main character and a thesis statement about the comparison between the brothers. For example:
The Outsiders is a novel which explores the life-changing effects of poor decision-making on Ponyboy, the narrator and his friends and family, all of whom are members of "The Greasers," a local gang. Ponyboy is the youngest of three brothers, Darry, Sodapop and himself, and it is their influence and other external and internal factors that lead this story to its tragic end. In the absence of any parental guidance, each brother plays a role in creating and solving the problems which face Ponyboy.
The second sentence could be the thesis statement and the third sentence could be the transition into your body paragraphs which will them make the comparison that is required in terms of the question. The transition sentence is the link and ensures that the essay will flow in the best direction to its logical conclusion. In your conclusion, recap on the contents of your introduction which will ensure that you have answered the question. Be sure that there is no new information in your conclusion. It should basically mirror the introduction.
Refer to the links below to help you with your character comparison and other relevant information.