Hi or hello is fine. :)
Good morning/afternoon/evening if you want to be more formal.
Strangely we don't tend to say 'good day' often. It sounds a little stiff and old fashioned.
Also in English (or at least Australia) we tend to follow it up with a "How are you?" type question.
"what's up?", "how are things?", or "what's happening?" are common casual expressions. And after giving a (generally short) answer you'd usually then reciprocate with a similar question.