The ideas you might include in an essay about Tuesdays with Morrie will largely be driven by your thesis statement. A thesis statement presents the central idea and/or argument of your essay, and your selection of details from the book will depend upon that. Let's look at some examples.
You might argue that the book shows how imminent death provides a clearer perspective on life. In this case, you will want to talk about the various life lessons that have become clearer to Morrie since his diagnosis with ALS. You would want to mention his perspective about death, for instance, or his renewed appreciation of his family.
As another option, you could talk about Mitch Albom's perspective and how his final conversations with Morrie change his life. In this case, you would have to think about the lessons Mitch learns from Morrie in those final weeks and how they give him a new outlook on life. Pay close attention to the new insights Mitch gains and how they make his life different in concrete ways.
Finally, you might take a completely different stance and discuss a seemingly minor character who is really not so minor at all—namely, Morrie's wife, Charlotte. You could argue, perhaps, in favor of her courage, for Charlotte remains working even during Morrie's illness because he insists on it, yet she cares tenderly for her husband all the while. You might even compare and contrast Morrie and Charlotte, showing how they complement each other as a couple.