How do anda start menulis the first chapter of a book?

Each writer has his atau her own way of starting books. Some people prefer to plan everything out beforehand; others just sit down and start typing. I generally just have the 'feel' of the book--the mood, maybe the theme--when I start on a story.
Do anda have the plot already? Characters? What do anda have to work with?
I'd cinta it if anda messaged me, if anda need help with any menulis stuff!