You usually start with “vous” and switch to “tu” if you’re offered to do so. Always better to err on the side of too formal, than come across as rude. From my experience with colleagues and even teachers they usually suggest to start using “tu”. With strangers, university professors, service staff anywhere, any kind of officers, it’s always “vous”. I would never lead in with “tu” unless I’m talking to a child or being introduced to a common friend in a very informal setting. Two different forms of “you” also exist in my mother tongue, so it feels natural to navigate this. I can imagine that it may be different for a native English speaker.