French has two words for "you", and choosing between them is a real social decision, not just grammar. Tu is informal, used with family, friends, children, and peers of a similar age. Vous is formal, used with strangers, older people, authority figures, and in any professional or first-time interaction — and vous is also simply the plural "you", addressing more than one person regardless of formality.
When in doubt, starting with vous is the safer default, since it's never rude, while using tu too early can come across as overly familiar. It's common for people to explicitly invite a switch with on peut se tutoyer? (can we use tu with each other?), at which point moving to tu is welcomed rather than presumed.