An essay about critical thinking could cover any number of topics. What you could or should write depends greatly on your interests and on the exact wording of your paper topic. Some possible things to write about would be:
How critical thinking is different from other kinds of thought. This would be an essay in which you try to define critical thinking, saying what it is and what it is not.
How critical thinking can be beneficial in particular circumstances. In this sort of essay, you would discuss how critical thinking could help you succeed in some particular aspect of your life.
Barriers to critical thinking. This essay would discuss the reasons why people find it difficult to think critically. It might propose ways to teach critical thinking skills.