This is an important topic. Here are some points to get you started.
First, you can talk about the dangers of stereotyping. There are real dangers as we look at history. For example, you can talk about the many pogroms and genocides that have taken place in the history of our world. These atrocious acts started in some ways through stereotyping. So, what might seem innocuous at first can lead to the most serious crimes.
Second, you can also write about how stereotyping still exists today. Too often we make judgments on the externals and for this reason do not get the the heart of the matter. Think of the media and the movies we watch. You might even want to think about the famous speech by Dr. Martin Luther King. In this famous speech, "I have a dream," he speaks about judging people by the content of their character rather than the color of their skin. He is getting at racial stereotyping.
These two points should you to started.