I suggest you begin with a rough outline, not right off with the introduction. After all, how are you to write an in introduction to a paper that you haven't written yet?
List all of the main topics you absolutely want to discuss in the paper, followed by one interesting fact, detail, or other supporting information for each point. Once you have the content you want, then you can write a rough conclusion reiterating the main points you discussed in the body.
Once you have all of that completed, the introduction will be much easier. You can find a catchy way to lead your readers into the paper you have already written.
I hope this helps you get started.
Best of luck!