When we learn other Asian language like Korean and Japanese, we were told to talk with people in formal tone when we first met. Some of them decided to use whether formal or informal tone by ages. In fact, sometimes you won't know the age of others when you first meet, turn up you have to talk a few times in formal tone before getting know with new friends.
To put it simply, du is informal tone and Sie is formal tone. It is quite surprising that the local people not using formal tone much even for first meeting. That is quite depressing for me because the regular verb form is used for formal tone. If I have to use informal tone all the time, I have to be aware of the verb form changed, like:
wohnen: live
Wo wohnst du -> Wo wohnen Sie
for verb, -st is used for du mostly
However, we don't really talk such a short sentence all the time LOL
sprechen: talk
Welche sprachen sprichst du
Welche sprache sprechen Sie
they always have "special cases" for some verbs...
German is wonderful language but confusing for language learners who their mother language don't have any "gender" or "forms"