The important thing with a conclusion is to reassert the most important aspects of your essay. It is best not to say anything new, as the space left will not allow for you to explain your new point. Instead, try to read through your introduction again, and remember the things you were looking to put at the center of your argument. These points are the best place to start. Example - if my introduction stated: 'I will argue that comedic elements used in Plath's The Bell Jar had a humanizing rather than trivializing effect'my conclusion might then read: 'I have shown that comedic moments such as a surreal scene with Buddy reveal the teenage protagonist's human responses to be regular, but so honest that instead of trivial they seem profound'. Try to keep the conclusion precise, and limit yourself to summarizing the argument you have already made.