When do I use tu, when do I use vous?

First of all, you need to ask yourself the question: am I addressing one person, or more than one? If the answer is more then one, simple! You ALWAYS use vous because this is the plural form.

If the answer is just one person, things are a bit more complicated because you can use tu or vous, depending on the situation. Tu is informal, and as a general rule you use tu when addressing people of the same age as you, or that you know well, for example your friends, classmates, colleagues, and members of your family. Vous is used when you want to show respect, or put a bit of distance between you and the person you'e talking to. Think: teachers, a stranger on the street, the person serving you in a shop, your boss.