You'll have to excuse the spelling (and the memory) but there are three ways to say hello: The morning greeing is Ohio gosai masu Konichi wa is used from late morning till early evening. The evening greeting is Kon ban wa.
But your question was "How are you?" and as Devil Cupcapes pointed out, that's "O genki deuka".
The response is "Hai genki desu. Anata wa? To which, "Hai genki desu."
(They're not called the Hai Society for nothing.)
When I worked for the Japanese I managed to abbreviate the ritual to "Pretty genki, thanks mate. How about you?"
Or if I was feeling very formal, I'd say "mate san".
** New record claimed: the Ask spell checker says I made 16 errors there. **