How do I write a long introduction (thesis statement) as the first paragraph for the topic "Gap between supposed authority and reality"?

It can be tricky to write a good introduction, but writing an introduction can help you think through the rest of your essay before you begin writing.
The first thing you need to do, as you mentioned, is to come up with your thesis statement. Since your topic is "gap between supposed authority and reality" a good place to start is by re-wording that into an opinion. You can first try making it a yes or no question, such as "is there a gap between supposed authority and reality?"
Then decide if you want to answer "yes", "no", "yes, but" or "no, but". Answering yes or no is a good idea because it makes it easier to keep your writing focused, and an essay will be much more convincing if you don't waiver on your point.
The next step is to turn the question into your answer. This could start with something as simple as "There is a gap between authority and reality." And then you can rephrase to make it more unique and tailored to your individual voice.
Once you've got your thesis, I like to think of three points of evidence. What three points am I going to make to show why my thesis is correct? I start with very general ideas, and then my next step is to go through the story and find as much evidence as possible to defend each of my points.
I would suggest finding your evidence and thinking through your points before you begin writing your introduction, since it will help you be sure your essay is concise and your intro fits with everything you plan to say throughout the rest of the piece.