When do I use "du" vs. "Sie"?
du (informal): Used for friends, family, children, pets, people your own age you know well.
Sie (formal): Used for strangers, people in authority, older people, in professional settings, and anyone you don't know well or want to show respect to. "Sie" is always capitalized.
Example
To a friend: Wie geht es dir?
To a stranger: Wie geht es Ihnen?