The pronoun "tu" is used in informal contexts. Situations where it should be used include:
when you are speaking to someone in an age group close to yours
when you are speaking to someone you know well (friends, family)
The pronoun "vous" is more formal. It is used when:
you are speaking to someone who is significantly older than you
you are speaking ot someone who is "superior" to you hierarchically (a colleague/boss, teacher)
someone you don't know at all or just met
When in doubt, use "vous" until the person you are addressing allows you to use "tu".