Naturally the introduction comes first--but that doesn't mean that it has to be written first. I suggest that you go ahead and write whatever is on your mind and don't worry about the order. You can write the introduction last of all, and you may find it easier to write it after you have seen what you have written in your essay. You can't make an omelet without breaking a few eggs. Learn to be a little bit messy when you're writing an essay. (Of course, you can't do that when writing an essay in class.) Don't get hung up searching for a word; leave a blank and think about the right word later. You should always figure on writing at least a couple of drafts, with the final draft a polishing and reorganization of the earlier versions. I can't tell you how to write an introduction to an essay on suspense without knowing what you intend to say about suspense. Maybe if you write a conclusion you can rephrase that and use it for your introduction.