1. Choose a topic with your assignment's source requirements in mind
Your assignment specifies that you need to find and use a case study. In that case, there are a lot of topics you WON'T want to choose, because many topics are not written about in case studies. For example, you probably won't find an engineering case study about:
how to tend sheep
2. Make sure your topic is not too broad
If your topic is a single word or simple phrase, it's probably too broad, like these topics:
the Eiffel Tower
When topics are too broad like this, your writing ends up turning into a long definition. Instead, topics should investigate or analyze something.
Sometimes it's hard to get from a broad topic to a research question that investigates something. To get started, think about your topic in these terms:
Who is involved or affected?
What problem would you like to resolve? Or, what is a possible solution?
When is this topic relevant? Right now, or more historical?
Where (geographically) is the topic relevant?
How did it happen?
Your topic doesn't need to be stated as a question. It could be a statement or hypothesis to find evidence for - or against.
3. Make sure your topic is not too narrow
It's possible to choose a topic that's so narrow, obscure, or weird that NO information sources exist on it - or, at least no case studies. For example:
What percentage of commercial airline crashes was traced to negligent maintenance during the ten years immediately preceding and following deregulation of the airline industry?
That example demonstrates another problem: the answer would simply be a percentage, and you wouldn't be able to stretch that into an essay.
You may need to do a preliminary search on your topic to make sure you can actually find a case study about it. As long as you start the assignment early enough, you should be able to find a topic and a case study no problem!