When I first started out at about 13-14, I think I was basically just copying stories I had already read. My first comic is basically Sailor Moon, my second was sorta like Card Captor Sakura. I slowly but surely started making content that was more original.
The idea for my first full length comic, True Power, came to me in a dream. I was flying around on a flashlight. When I woke up I thought that would be a good idea for a comic. I built the story around that.
For my newest comic, Celestial Chronicle Shion, I wanted to do a more serious, darker comic and I wanted to do something with the occult in it. When I thought of a setting for it, I thought that occult in space would be fun. And then since it was in space there would also need to be aliens so now I have two comics in a row that feature aliens!
I now have a third comic idea in my head, but it's just still a spark in my head. I haven't done anything for it since I already have two comics going on and need to focus on them.