The most important thing here is to pick a topic that is of interest to you. You are most likely to be able to write a good essay if you actually care about the topic. Given this, it is likely that you would be able to do a better job of picking a topic than we could.
Let me suggest some examples of what you might pick if you have an interest in various general areas. If you are interested in the social sciences, you could write an essay describing what you think are the most important causes of poverty or discussing how it could be eradicated. You could write an essay on what you think is the most pressing problem that faces the United States today.
If you are more interested in the arts and literature, you could write an essay on what role from Shakespeare (or any other playwright) you would most like to play. You could write about what your favorite painting is.
If you are interested in the sciences, you could discuss what scientific discovery has made the biggest impact on human society. Or you could write about what you think the best solution to reducing America's energy consumption would be.
One way or another, you need to write about something that you care about. It will be much easier to write a good essay if you have a topic that interests you.