As soon as you address somebody by his or her first name, then you would use _du_:
> e.g. Hallo Peter, wie geht's dir?


On the other hand when you call a person by the surname, you would use _Sie_:
> e.g. Frau Müller, können Sie mir helfen?

note: always use a capital lettre with _Sie_.


**Edit: What I wrote above is the usual case, meant as a direct answer to the question, there are some exceptions to this rule however.**