The purposes of an introduction in this kind of paper are (1) to get the reader's attention, (2) to convince the reader that the topic is worth further consideration, (3) to create, or re-create, the image of an ongoing conversation on the subject, (4) to state the author's position on the subject, and (5) to preview the rest of the paper.
There are many different ways to get the readers attention. One way is to open with a narrative of some kind, an example, or even a quotation. Another method is to pose a problem to be solved, a question to be answered, or a paradox to unravel. But it is also possible to get attention by coming right out with your position (your claim or thesis). Such abrupt introductions are unusual, and it is partly because they are unusual that they sometimes work. One technique that is best avoided is to "be cute" in the sense of posing in the role of the all-too-clever college student making a comment about the writing process, assignment, or situation. If you use an introduction that draws more attention to you as a writer than to the subject of your paper, then you have failed.
Not all readers want to read everything that is placed in front of them. Some subjects just aren't "interesting" to some readers. The second purpose of an introduction is to make the reader interested. Interest may be defined as the reader's believe that the text is somehow pertinent to her or his life, goals, or projects. Therefore, to create interest, a writer tries to show--without being to overt about it--that the subject matter is of some consequence to the reader's life.
The third purpose is to create the image of an ongoing conversation or debate on the subject. This section of a research paper introduction is sometimes called the lit review. In it, the writer summarizes the opinions of others, often in a chronological sequence. It is important that this section be more than a string of summaries without a theme. As you summarize the opinions of others, you should emphasize those aspects of the original that contribute to the issue you wish to address. Think of the issue as a point of contention within the larger subject. If you are summarizing articles on cats, but you wish to address the use of cats in vivesection, then want to summarize only those parts of the things that you have read that contribute to this focus. As the lit review develops, it should point to the need for someone else to say something about the subject--that someone else being you, the writer. In other words, set yourself up setting the stage that prepares the readers to hear your contribution the question left hanging by the lit review.
The fourth purpose is to state your thesis or claim. It is important to make clear what you are setting out to prove in the rest of the paper, but the sentence that does this does not have to follow the lit review. It could be any place in the introduction as long as it is a place of emphasis that clearly attracts the reader's attention. A thesis is a sentence that makes a claim about a subject and then often explains the basis for the claim in a subordinate clause. For instance, "Galileo was unsuccessful in portraying an orthodox ethos in The Letter to the Grand Duchess Christina because he not only argued for a new view of the solar system, but he also tried to diminish the stature of theology as a discipline." That sentence goes out on a limb: it makes a controversial claim that needs to be proven, and it gives the basic grounds supporting the claim.
Finally, an introduction often previews the rest of the paper, sometimes bluntly (In support of this claim, I will first show that . . . and then that . . . and, finally, I will draw out the implications), sometimes in a more subdued manner (Whenever questions of these kind arise, it is important to consider this, and then that, and finally the other thing).