Sometimes it's easy to feel it out:
Meet a random old lady on the street: Vous
Talk to a small child: Tu
In a job interview: Vous
Among friends of friends at a party: Tu
But often you run into gray area, which is what I face at my schools. The principle vous me, I vous him, it's all good. But the teachers all tu each other, and they all tu me. I started off by vousing all of them, and only one of them specifically told me I could tu him and all the other 4th grade teachers. Problem: I deal with 17 or 18 different teachers a week, I have no clue who teaches fourth grade, and during that first week I didn't want to deal with remembering who I called what. At the same time I would have felt weird saying vous to some of the teachers and tu to the other ones. So I did what I probably shouldn't and kept right on vousing him and all the other teachers, even though it's starting to feel weird and way too formal, and it makes me something of an outsider. Today another teacher told me to tu her... shoot I don't remember which one it was.
English, why you no prepare me for this?