First, you need to choose what aspect of the English language you wish to cover in your essay. One possibility is the history of the English language. You could focus for example, on how one or more invasions, such as those of the Romans, Vikings, or Normans, affected the syntactic or semantic nature of the language. Another possibility would be a synchronic essay on the diversity of world Englishes. Another area which might be related to the academic skills you are developing in English classes is the difference between the grapholect of standard edited English and various forms of spoken or informal written English.