What should i do to expand this novel idea? :P?

A bad person needs to be in this story(like all books or movies)and later in the story Rebecca needs to be a main character.Like she needs to be A princess that can change form into a human and a mermaid.She need to fall in love with a human boy,who is going to change into a merman because of their love for each other.August needs to die by getting killed by the bad person who is the King of bad mermaids/mans.I hoped i helped you or and here's my email:porsharobinson@ymail.com