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.