You have asked a massive question that is very dependent on what your actual essay title is. If you have just been told to write an essay on this novel you need to think about what theme or approach you are going to take. I have included a link below to the enotes section on themes and other criticism that hopefully will give you some ideas.
If you have been given a title, you need to work out your thesis, or what your general answer is going to be to the question. Any introduction, generally speaking should do three things: it should start with an attention-grabber, linking the theme of what you are going to talk about to the present day, it should then state the novel and author that you are going to talk about, and lastly it should state your thesis and how you are going to argue it.
So, without more information about the exact essay title you have been given, these are the only points I can give you.
I have to compare it with Of Mice and Men :/
It depends on the subject of the essay, does it require you to explain the themes used by George Orwell or does it require you to explain the characters and their Russian Revolution counterparts. However it is best to start off with a thesis that explains what you are going to furthur elaborate on later in you essay.
Example: Explain Themes, topic sentence
The novel ‘Animal Farm’ by George Orwell was conceived and written as a Satire, using scarcams and irony to convey the deeper meaning of the story.