Before you start writing the essay, take a separate sheet of paper and write down the main points you want to make. In fact, write down the first sentence of each paragraph you plan to write. This will give your essay a feeling of shape, structure, and that the reader is in the hands of someone who knows where she is going.
"Tell them what you are going to say, then say it, then tell them what you said." Have an introductory paragraph, then the body of the essay, then a summary. The introduction and the summary don't need to be long or fancy, but they should be there.
If you express any opinions, support them by citing facts. Don't waste time trying to make your opinions sound good or authoritative or literary, use the time to support them.
Don't try to make the writing fancy; concentrate on making it clear.