To address someone with the informal "you" One of the most confusing things about conversational French is figuring out whether to address someone with the formal (and/or plural) "vous" or the informal and/or familiar "tu." Generally speaking, you use "vous" when you don't know someone, or when you're addressing someone older or of higher authority than you, or when you just want to show respect to someone; you use "tu" when you're familiar with someone, or when you're speaking to someone of your age or younger. Families differ on if kids should tutoient or vouvoient their parents, their grandparents, their aunts and uncles, etc.
That being said, my first day at school was made much more comfortable by the mandate that all teachers and staff members addressed each other as "tu," even if you didn't know the other person that well. It's like a little family, a team; it made me feel very welcome. On the flip side, I always address my landlords as "vous," even though they've invited me to their house for multiple meals -- what's nice is that they also address me as "vous." Business arrangement first and foremost, I guess.
The cool thing is when these forms of addressing people start to shift. A "vous" to "tu" shift (it never happens the other way around) means friendship, or at the very least, familiarity. I've gotten permission to tutoie Bernard, who runs the bookstore, who I've spent a lot of time with. Just today, the boulanger (bread man) who comes around to our neighborhood said that I could of course tutoie him. I'm pretty sure I'm a few weeks away from permission from the sausage man at market and the madame who runs the grocery store to tutoie them. I may not be able to fit in seamlessly to French life, but I'm becoming accepted by those who live and own that life.
On a side note: incidentally, I'm apparently not only becoming accepted by, but attractive to the French, as well. At least the males of the species. My CE2s (3rd graders) made a big deal about two of the boys in the class having crushes on me. Fortunately, they're 3rd grade crushes. I think I can handle that. What I wasn't prepared to handle were the advances of a couple boys my age, maybe a bit younger, maybe a bit older (I can never tell with the French), outside a bar at the edge of town. They're young, immature, they didn't quite know what they were truly attempting. I rebuffed their advances not only in French, but like the French do, with a certain aloofness and gentle snide comments (which may not make sense until you hear some native speakers go at it). My French must be getting pretty good...