Echo or restate the main points of your introduction and thesis statement in your conclusion. For example, if the three main challenges you faced moving to a new country were learning a new language, making new friends and adjusting to a different culture, write, "When I moved to the United States three years ago, I did not think I could learn a new language, make new friends, and adjust to the culture." In your concluding paragraph, mention the ways you learned the language, and perhaps some of the friends you made who helped you adjust to your new country, unifying the essay.
Perhaps you have been assigned a comparative essay in class, or need to write a comprehensive comparative report for work. In order to write a stellar comparative essay, you have to start off by picking two subjects that have enough similarities and differences to be compared in a meaningful way, such as two sports teams or two systems of government. Once you have that, then you have to find at least two or three points of comparison and use research, facts, and well-organized paragraphs to impress and captivate your readers. Writing the comparative essay is an important skill that you will use many times throughout your scholastic career.