Here is how to make subtopics for your paper or project!

Make a list of facts and questions that you have researched for your topic. This can also include questions that you thought about when you started your topic.

Now, look at all those questions and sentences - find words, phrases, or ideas that are similar, and begin grouping those facts together.

Write out the headings you have made - the similar things - and list each question or fact under the heading where it belongs.

This excellent information is from the website linked below, which is from Cambridge Rindge and Latin School - they even have a list of common sub-topic words to usel:

For example:
Say you have to do a project about local water pollution and you pick the Charles River as a body of water to research. Your general topic in this case is "Charles River Pollution".
First, you may have brain stormed some questions like the following:

• How much is the Charles River polluted?
• What causes pollution in the Charles?
• Does the government do anything to clean up the pollution?
• How does the pollution in the river affect plant or animal life?
• How does the pollution in the river affect people?
• Will I ever be able to swim in the Charles?

Next, you can identify and choose some categories these questions fit into, and select words that label those categories. It helps to think of these words as part of a phrase that includes your general topic. Keep reading and this will become more clear.

Statistics How much is the Charles River polluted?
Causes What causes pollution in the Charles?
Solutions Does the government do anything to clean up the pollution?
Effects How does the pollution in the river affect plant or animal life?
Effects How does the pollution in the river affect people?
Future Will I ever be able to swim in the Charles?
Now, turn those categorizing words into subtopic headings by linking them to your general topic which is Charles River Pollution.
How? Write your subtopic headings like this, leaving space under them for your questions:

• Statistics about Charles River Pollution

• Causes of Charles River Pollution

• Effects of Charles River Pollution

• Solutions to Charles River Pollution

• Future of Charles River Pollution

Now, you can rewrite your questions under these subtopic headings.
LIKE THIS:

• Statistics about Charles River Pollution
  How much is the Charles River polluted?

• Causes of Charles River Pollution
  What causes Charles River pollution?

• Effects of Charles River Pollution
  How does the pollution in the river affect plant or animal life?
  How does the pollution in the river affect people?

• Solutions to Charles River Pollution
  Does the government do anything to clean up the pollution?

• Future of Charles River Pollution
  Will people ever be able to swim in the Charles River?

Now you can add any new questions you can think of under the subtopic headings. You are now on your way to making an outline. Use the Common Subtopic Headings list to give you ideas for categories. It can also help you to develop questions!