One question people who haven't written a book ask is "Where do you get your ideas?" My answer is that there are many ideas. Getting ideas for stories isn't hard. They're everywhere. Using those ideas takes some consideration.
The idea appears and triggers a reason to use it in a story. I've had many ideas but not all of them have developed into a story. The strangest idea I ever had was what is someone could see and read the meaning of lines of fire on a person's skin. Somehow I knew I wanted this to become a story told and it did.
Not I had the intention and the next thing was my attitude toward my desire to write a story. After thinking this over, I decided the lines of fire went to a person's good or evil. Could the lines be tainted and what did it mean if they were. So now I had another level to my idea.
Then came a real question. What was the purpose of my story. The world I envisioned began to evolve and the purpose was found. This purpose was to show how power can be corrupted and the effect it has on the main characters in the story.
Another step in using the idea involves research and was I willing to research what was needed to create a new world. Part of the research came from my imagination, and delving into where the idea would lead me.
Knowing the people is another way to develop the idea. I looked around me and I studied people trying to find those who had the qualities I might need in the characters. Here I also used my interest in Astrology to develop the characters.
So while there are hundreds of ideas for stories out there, some of these ideas aren't useable for my but they might for another person. So when you have an idea explore it before you start writing the story.