"Γεια σου" or just "Γεια" (less formal).
If you want to be more respectful and formal you may say "Γεια σας" (plural).
Here is how γεια, γεια σου, γεια σας sounds in Greek:
https://learn-greek-online.com/mp3/geia-geiasou-geiasas.mp3
--------------------------------------------------------------------------------------------------------------------------- Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.