Barb's advice is excellent - write your introduction last! I write for a living, and in the early days I remember getting "stuck" at that opening sentence/paragraph. While fretting over what to write, I'd forget all the great Ideas I had for the rest of the article. :-? So work on the body of your essay first, then re-read it and decide the best way to introduce it. Sometimes one sentence is enough, sometimes you'll need a brief paragraph.