A simple model for the relationship between the introduction, the body, and the conclusion is the old newspaper maxim:
"You tell 'em what you're gonna tell 'em you tell 'em, and then you tell 'em what you told 'em."
In an introduction, you lay out a plan for what will follow. However, there is more to writing an introduction than merely summarizing the points of your essay; you must find a way to open discussion of the topic. There are several ways to do this, but a simple and effective method uses the analogy of a triangle.
Imagine an inverted triangle, thus:
In this model, your introduction begins with the general and moves toward the specific, as the sides of the triangle narrow toward a point. Ask yourself how the specific question you are addressing in the essay relates to a greater issue or field. For example, if you are writing about how Waiting For Godot subverts traditional notions of plot, you might want to begin by explaining what a traditional notion of plot is, or by discussing the characteristics of Beckett's work in general.
The question you take up in your essay does not exist in a vacuum; it arises out of a greater set of concerns. Your introduction can provide this background so that the reader is not coming to the discussion cold: ask yourself what your audience knows already, and what it needs to know in order to understand the context for your thesis.
By the time you reach the end of your introductory paragraph, you should be ready to state the thesis of your essay. The introduction need not give away all your opinions and conclusions, but you should give your reader a clear idea of what you will be discussing. For an example of the inverted triangle structure, read a sample expository essay written by a UVic student.