Try writing a simple, short story about some classic fluffy canon like getting adopted as a pet, learning to use the litter box, and eating spaghetti. How you go about doing these classic things will help define your “headcanon” as they call it. Once you have written a few warmup stories and start to get a feel for your characters, their environments, motivations, and personalities, you will be able to write more complex material the community will like.
You can use the established cannon fluffies listed above or make your own like i did here for laughs! Just have fun with it!
And welcome to fc! 
