Anyone that has wanted to write a series, be it novel, comic, tv or film should begin with establishing the world along with the protagonist or main character. To do this, it requires that you create what I call the Guiding Principles of your world.
THEME: The main idea of your story, best expressed as on word.
PURPOSE: The reason your story must be told (The important message)
EXPECTATION: The result of applying the PURPOSE.
LOGLINE: A brief description of the main character and conflict in your story.
Once you have sorted this information, then you can consider the Audience and Genre.
If you cannot express your story using these four basic concepts, you will have a difficult time pitching your story to others in a concise manner. I have provided a GUIDING PRINCIPLES worksheet for your use.
Here is a link to my YouTube channel that takes you through the entire process from Guiding Principles to the first draft of your story:
Contact me if you have any questions. Happy Writing!