What happens next?

How to write a book with another person
Decide who you're going to write the book with.
It should be someone you get along well with, someone who likes to write, and someone who has a similar writing style to yours. Pick somebody that is open to new ideas and isn't bossy.

Pick from: [-] You need a person you can trust and who you'll want to share your ideas with. Find someone you can discuss ideas with easily and who values your opinion. [-] Pick a location (perhaps in a park, forest, or somewhere other) and work out a plot plot. Don't make your character's house outlandish or flashy. [-] Only decide who you're going to talk to about your writing, and make sure this person is willing to listen to your arguments. This is not a good choice if you're planning to write your book by yourself. [-] If you find yourself too tired, you could give this person a cookie after every reading of the book. Give people the choice to take turns reading.
You need a person you can trust and who you'll want to share your ideas with. Find someone you can discuss ideas with easily and who values your opinion.