That depends on what the rest of your essay is about. You have to take the main points you're trying to make and combine them into one sentence. This is called a "thesis" or a "hook."
The sentence has to sound like you, so I won't make it for you. Plus, if it doesn't and your teacher decides to search your intro in Google or something, she'll get right to this question and know you cheated.
*paranoid*
Anways, here: http://www.ehow.com/how_5127257_write-hook-essay.html