Stuck in my head

I have this idea for a story.  I call it Sex in the city meets Lizzy McGuire meets Inside Out.  Again, it's stuck in my head. I have small parts of the characters.  I guess sometimes that's where you start.  That's a dream of mine.  I would like to write a book.  Truthfully, I am petrified to do it and perhaps why stories stay stuck in my head.  For some reason, while this one is technically still stuck in my head...it's taken a life if its own and may have some headway with this one.  We'll see.