Read or listen very carefully to the given instructions for a writing assignment. Make sure that you know what is expected . Ask questions if you are unsure.
Note the expected length (number of words or pages) of the writing.
Then consider your area of interest in choosing your subject. For example, let's say you wanted to write about the island of Crete. It's a small island to the south of Greece, but could this be covered in a "small way"?
That is to say, would you be able to cover the topic in the number of pages your instructor has assigned.
Create a bullet list of things associated with the subject. (Google it or look it up in Wikipedia.) Then, think again about whether your subject fits the assignment.
It is always better to follow your interest (within the guidelines of the assignment.) This will make your research and your writing more enjoyable. But be practical. Focus the subject to a topic that you can cover.
A topic is different from a subject. A subject is broad (general). A topic is more focused and manageable.
For example, break down your subject further:
The Minoan People
Before you decide to stick with a topic, do a little research on the Internet or at your local library. Which topic has more resources available? Read a little so that you can be knowledgeable on the subject. Then, adjust your focus further.
Do a bit of background reading.
Anyone can copy details from an encyclopedia and simply state facts. But what will the added value of your writing be? Your attitude or opinion is what makes your writing different from the writing of everyone else.
Questions to ask yourself:
The angle of interest is called the controlling idea. It includes the writer's opinion on or attitude toward the topic. The reader can agree or disagree with it.
Broad subject: An ancient civilization on Crete
Narrower subject: The Minoan palace of Knossos
Focused topic: The frescoes of the palace of Knossos
More focused topic: A fresco in the palace of Knossos
Controlling idea: reveal(s) the lifestyle of the Minoans