In the French language, saying “tu” is the familiar way to say you (singular) and “vous” is the more formal, polite way of saying you (singular). “Vous” is also the word they use for the plural version of you.
In English, as you obviously know to be reading this, we don’t differentiate between the plural, singular, formal, or familiar when we say “you” but, in the French language, they do... (tu and vous)...and the decision as to which to use has certain social nuances and ramifications.
It would be very helpful for you to learn the French language rules to using tu and vous:
The rules are rather simple, so they are definitely worth brushing up on before heading to Paris.
When in doubt...
If you are ever in doubt, err on the side of caution and use "vous" ...but that is not to say just use vous all the time!....because if you use "vous" in a situation where you really shouldn't, such as just after someone has told you can tutoie them, or you are a young student amongst other young students, then you are basically saying, “No thanks, not interested in being on familiar terms with you."
Even if the other person knows you are probably just not catching on, there will still be at least an awkward moment. Remember: French people have been speaking and hearing French all their lives, so the nuances of this 'tu-vous' thing is deeply ingrained. It’s part of their language and culture.
Think of it like it as similar to if an old man who didn’t speak English very well thought that “hot babe” was just another way to say “pretty girl”. Well, I’d bet you’d feel discomfort hearing him tell a 6 year-old girl she looks like a hot babe even if you knew he probably hadn’t caught on to the nuance of that expression. He may have said it innocently, but it would probably nonetheless affect your view of the man or, at the very least, create an awkward moment...
...Sorry, a bit of a yucky example, but it demonstrates well the power of nuances for a person’s own mother tongue...you just can’t escape them. So as much as you think the tu-vous question should be no big deal, you can’t control the effect of the nuance on French people. It’s ingrained in them.
Trust me on this one... to learn and practise these simple French language rules regarding ‘tu’ and ‘vous’ will help you very much with your social interactions with Parisians....even if just in making inquiries on the street for directions and such.