What is a critical essay? Answering this is your first major task. You must understand the assignment, and the first key to understanding the assignment is getting a good grasp of what the instructor means by "critical essay." Be SURE you understand this. Keep asking questions until you do.
Usually, a critical essay means a short paper discussing YOUR analysis (pro and con, good and bad, well done and badly done) of the various aspects of a work of literature or a piece of art or a topic. Be sure to find out EXACTLY what your topic is. Don't leave class until you are sure.
Once you know your topic, you need to do some background reading. Hopefully, your topic is narrow and specific and you can read most of what has been written about it.
Once you have read everything about it, you should have a grasp of the major issues surrounding the topic, any questions which have arisen, the major authors in the area and their viewpoints. You should discuss these things from YOUR perspective.
Which critics or authors make sense? Which sound like bunk? You should give clear, reasonable arguments for each conclusion you give.