While in English we are happy to use just one “you” to convey the idea of who we are addressing, in many other European languages there is more than one word for you. In French, we would use “tu” for informal occasions and “vous” to be more polite, or to speak to a group of people. German uses “du” for informal situations, “ihr” for a group of people you know well and “Sie” more formally, whether singular or plural.
But is it really as easy as thinking “Do I know the person well or not?” Often it is not so simple and you may need to ask yourself a range of questions regarding relative power and social standing. For example, one would almost always use “Vous” or “Sie” for one’s manager or teacher, but also with colleagues one knows quite well. Children would often be addressed as “tu” or “du”, whether one knows them or not.
As a general rule of thumb you are much less likely to cause offence if you are more formal rather than less formal. We would recommend always using the formal forms of address until the native speaker invites you to do otherwise.
On that note – did you know that both French and German have specific verbs for “calling each other by the informal you”? In French it is “tutoyer” and in German “duzen”. So, if a French person says “on peut se tutoyer” then you know they are inviting you to use “tu” – now all you have to do is remember to do so!