What is a good introduction for the novel Animal Farm surrounding the topic of corruption of power?

There are a number of ways in which you might write an introduction for this essay. Firstly, you could explore the key words in the title and thereby establish the parameters of your discussion in the main body of the essay. For example, there are lots of different types of power in Animal Farm. There is the physical power represented by the dogs, and then there is the power of propaganda, represented by Squealer. There is also the power of ideas, represented most of all by Old Major and the speech he gives to the other animals in the opening chapter. You might choose in your essay to focus on one or two types of power, and the introduction is a good place to signal to the reader what your specific focuses will be. The other key word in the question, "corruption," broadly means the debasement of something which was good. That might encompass moral corruption, ideological corruption, or even physical corruption. Again, in your introduction, you might introduce the specific types of corruption that you will be focusing on in the main body of the essay.
A second option is to set up some of the debates that you might deal with in the main body of the essay. For example, you might touch upon the question of what type of power corrupts most or upon the question of whether power corrupts all types of people (or animals) equally or some types of people more than others. Introducing a few broad ideas about debates like these, which you can go into in more detail in the main body of the essay, is always a good way to begin.
A third option is to outline some of the contextual influences behind the book. For example, you might look at Orwell's time spent fighting for the Republican army in the Spanish civil war and trace some links between that time in Orwell's life and some of the ideas about power in the book. Alternatively, you could of course make some links between some of the people (Mussolini, Franco, Hitler, Lenin, Stalin) and organizations that exercised power in Orwell's life and the characters who exercise power in Animal Farm.
I hope one of these ideas appeals to you. Good luck with your essay!

One might begin such an introduction by stating that Animal Farm is an allegory, a representation in metaphorical terms of an actual series of historical events. The takeover of the Manor Farm is symbolic of the Bolshevik Revolution of 1917. What at the time was greeted by socialists worldwide as an epic event that would eventually transform the whole world deteriorated eventually into the cruel dictatorship of Joseph Stalin. In Orwell's story, the animal leadership start out with apparently good intentions and egalitarian ideals. The problem is that a power struggle inevitably occurs between the leaders, Napoleon and Snowball, and the former, the more ruthless of the two, wins out. In contrast to the stated ideals of the animals, Napoleon begins to control and to dominate the others, since the fact of exercising force in the first place (which was necessary in order to accomplish the Revolution) has corrupted him. It is as if a chain reaction or a self-perpetuating process has occurred in which those who wield any degree of power will seek to become still more powerful, causing the whole movement for equality to self-destruct.