Subtopics that support a main idea vary based on the subject of your main idea. Subtopics represent smaller details that you can focus and elaborate in in your writing or organizational structure that all come together to make your main idea.
An example of a main idea and three subtopics would be:
Main idea- Bicycling
Subtopics #1- Bike Safety
Subtopics #2- Purchasing the right equipment
Subtopics #3- Effects of biking
I hope this helps! :)
