This advice is not for those who know no German at all, nor for those who speak fluently. So… for those in the middle:
in general, Sie is used for everyone except close friends, relatives and children up to 16-18.
The European is not as quick to assume a close relationship as the English/American. Thus, where an American very often uses the first name with anyone he has known for a short time, the European will more rarely use the first name, and most certainly never the Du form.

Colleagues and co-workers in an office will call each other Sie for months even though they see each other very often. If you are not totally right which for to use, just ask your German friend or… use the excuse that you are Auslander!

In many occasions the Du form is used extensively, such as Fasching-parties, sport gatherings and many other forms of social entertainment.

Filippo