How can I make my book private so no one can see it?

All books are private, by default.
 

You, or another authorized user who is linked to your account, can share your book so that it's visible to family & friends or to the public.

If your book has been shared and you want to change it back to PRIVATE, then:
- click on your book
- click SHARE
- at the top right, find "Privacy Level", and click "Edit"
- pick "Private" and click "SAVE"
 
If you're under 13, your parent (or teacher) needs to do this for you from their linked account.