It depends whether you are in France or Canada. As far as I understand, in France it's polite to say "vous" to strangers or people who you owe respect (elders, boss, customer..). You say "tu" when addressing someone close to you such as family or friend. You also have to conjugate the verbs accordingly "Voulez-vous.." "Veux-tu.."
Here in Canada it's more common to use tu in almost any situation unless one is being extremely polite. Similarly, in Canada in English it would be unusual to call a stranger "sir" but you would say it if you were talking to a customer or dignitary. I once called my boss "Vous" and she said "I'm not that old, why are you calling me vous?!"
Hope this helps!