Firstly, your introduction should be included with your research proposal and broadly present the outline of your ideas, what you wish to explore or establish, and why you want to study that particular area. Don't be afraid to revise or update the introduction constantly and keep it in mind throughout the process as it will keep your paper on track.
The good thing about an introduction is that it should be the last thing you write and therefore will be clear about what you wish to explore as you will have already done it by then.
An opening sentence that grabs the attention of the reader from the start is the best way to get them involved in the subject matter and encourage them to read on. The could be an anecdote, a quote, or even a controversial statement you wish to explore in the paper and let's face it there's a huge range of controversial statements connected to Castro so this should be too difficult.
The introduction should also include a brief introduction to and review of any literature which has gone before which is relevant to your paper and use this to create a theoretical outline to you piece In addition to the last piece, you should provide your opinion on those theoretical debates included in the literature you use. This will work in conjunction with the literature review section and add even more weight to your work. You should also explain how your research will contribute to the theoretical understanding of the topic. As intricate as all this is sounding, you shouldn't give too many details in your introduction as you want the reader to discover the information through reading. Remember you don't want to give it away at the beginning so stay away from results or conclusions in this section..
Give an outline of your paper at the end of your introduction which can be as simple as; In Chapter 2 I will discuss my methodology, in Chapter 3 set out my result etc..
Finally, finish your introduction with a thesis statement. This reason behind this is because it leads into your paper and reinforces what you are going to speak about..
You can use this framework to compose an essay on all sorts of aspects of Castros life such as the Cuban revolution, his premiership, the presidency, the Bay of Pigs and even his later life. There really is a multitude of topics to cover with this controversial figure of history. Good luck.