Most authors write about their favorite things so you could do that. Or if you watch tv you could write a thing called fanfics where you take the characters of that show and put them in a situation created by you. I find those a bit easier because you don't have to worry about creating characters you can just focus on the plot. But what you could do to get inspiration is look up on the internet for writing prompts. A bunch will give you an idea and you can write a short answer to those until something strikes your fancy then you can expand upon that. Another idea would be to take an idea from a dream you had, I do that sometimes.
Hope that helps!
This should depend on what you like..
1.If you want romance:Maybe about the love between a girl and several boys in a school.This will be really interesting if you know how to write.
2.If you want action:You can write about the fight between 2 association-They both want to have something at the same time,such as a legacy,or something that can change their life...
3.If you want fantasy:A magical kingdom with fairies and demons,something like that...
I don't know,all depends on you.Good luck!