I want to start writing a book?

I love reading and writing I love animals and I read a lot of vampire type book. Can anyone give me a good topic or idea to write about in a book like 400-500pg. Mybe. So something I can write a lot back. I thought I could write a book Bout a girl who causes the death of her mother & her parents comes bak to haunt her and she soon realizes her parents death wasn't a accident. What do u think. Help!