When should I use "tu" and when should I use "vous"?

The basic difference between the two is that "tu" is used for one person (i.e. in the singular) whereas "vous" is used for more than one person (i.e. in the plural). However, there are some instances where "vous" should be used for one person. It generally comes down to a question of politeness. For example, to a teacher or someone in a position of authority, you should ALWAYS use "vous". Generally speaking, "vous" should be used for anyone older than you, with the main exception being family, as they are typically very familiar to you. Hopefully that has cleared things up for you slightly. just remember: "tu" is FAMILIAR and "vous" is FORMAL.