I am trying to write a book but don't know what to write about any ideas?

I am trying to give tu an idea-
"A world ruled por demons. A few humans are alive and have to struggle hard to live. Humans are being tortured and killed por the demons. A thousand año old prophecy about a demon hunter will be brought and then some team against the darkness will rise"

Paranormal mixed with romance?
Idk. Thats all I really write about. Lol... Sorry for not being a big help