Writing an introduction to a literary analysis essay is very simple if you effectively pre-write and organize the essay first.  To provide a specific example, let's use the following prompt from To Kill a Mockingbird:

Describe Scout's learning experience throughout the book.

First, brainstorm answers to the prompt using specific examples (and quotes where possible) from the story.  Once you've created a long list of ideas, begin grouping ideas into three categories.  These will become the sub-topics for your argument.

In this essay, three logical sub-topics which answer the prompt include: 

1. Scout learns from her own mistakes and circumstances.
2. Scout learns direct lessons from her father Atticus.
3. Scout learns indirectly from things said/done by other characters.

Assuming you have ample evidence to support each of the above sub-topics and have visually organized these ideas by using an outline or a graphic organizer, you are ready to write your thesis statement.  The thesis statement is a general answer to the prompt.  For this essay an effective thesis statement is:

Scout's learning experience throughout the book can be categorized into three distinct patterns.

You've already created your three subtopics, so to complete your introduction, you simply need to put everything together and create an interesting opening sentence to hook the audience.  I always advise students to identify the text and the author in this hook.  Here is one idea of a complete introduction for the example prompt:

In Harper Lee's To Kill a Mockingbird, Scout is a precocious character who learns many adult lessons at a very young age.  Her learning experience throughout the book can be categorized into three distinct patterns.  First, she learns through her own mistakes and circumstances.  Second, she learns direct lessons from her father Atticus.  Finally, she learns many lessons indirectly from the comments and actions of other characters. (Note the thesis statement in bold.)

Hopefully this example will clear up any confusion and help you pattern your own essay introduction with specific guidance.  Good luck.