We tend to nod our heads when gesturing "yes", and shake our heads when gesturing "no". In Greece, gesturing "yes" is a slight downward nod. Gesturing "no" is an upward nod of the head.
The following video by Greek Pod 101 teaches how to greet others in Greece, whether in an informal situation (with friends and family) or a formal situation. Along with a hello, friends and family greet each other by kissing on both cheeks.
hi, hello, goodbye
good day, good morning, good afternoon
You are now prepared to greet your friends and family in true Greek fashion.