The biggest mistake you can make when writing an essay is not having a clear thesis statement. An essay is essentially an argument; being wishy-washy defeats the purpose. If you do not know where you stand, how can you persuade anyone else over to your side? Coming up with a thesis statement can be tough, but it helps if you choose a topic you feel strongly about. Make sure you outline exactly what it is you think, and then try and find supporting reasons of why you think that way.