When do I use 'tu' and when do I use 'vous'?

'Tu' is a SINGULAR SUBJECT PRONOUN. This means that it will be used when you are addressing one person.

'Vous' is a PLURAL SUBJECT PRONOUN. This means that you would use it when addressing multiple people. However, in French, you also have the 'polite' form, which is used when addressing someone that you don't know or that you have a formal relationship with (teachers, employers etc.). So 'vous' is also used when addressing a SINGLE person formally.

SUBJECT PROUNS replace the subject of the verb in a sentance                                                                     e.g. in "Alice went..." Alice is the subject of the verb went. If you use a subject pronoun, you replace "Alice" with "she".

In french there are six SUBJECT PRONOUNS:

SINGULAR (used when addressing or talking about one person):                                                                      1st person: 'je' (I)                                                                                                                                                                  2nd person: 'tu' (you)                                                                                                                                                            3rd person: 'il' (he); 'elle' (she)

PLURAL: (used when addressing or talking about multiple people):                                                              1st person: 'nous' (we)                                                                                                                                                     2nd person: 'vous' (you)                                                                                                                                                       3rd person: 'ils' (they masculin); 'elles' (they feminin).