Choose a theme that interests you. If you are writing about the U.S. and you have always had an interest in war, you could write on a current or past war in which the U.S. has been involved. The same goes for sports, music or anything else that falls under your interest.
Read up on the theme until you start to form an opinion. This opinion will be the foundation of your argument; this argument will go on to be the point of your entire essay, so you need to be informed enough to write about it at length.
Create an argument that is clear, concise and controversial. It should be something that is up for debate, such as "the U.S. has not been involved in as many wars during the 20th century as it should have been," or the converse.
Write an outline on paper. It should be divided into sections, with each section's heading responding to your argument from a different angle. There should be at least two sentences after each heading giving an explanation of the heading and an example.
Write a rough draft, using the bullet points from the outline as a guide. The headings should turn into topic sentences while the subheadings should turn into supporting sentences.
Edit your rough draft to make sure it makes sense, has proper spelling and grammar and is written in an engaging style.
Make any necessary changes and rewrites, and turn in the essay.