May be it`s an another "untranslatable" expression

, I just don`t know , but what is the proper answer to this simple question? What people
really wait to hear from me in this case?
Now you're asking a pretty advanced question.
There is no standard answer to this greeting. When I hear it, one option is to not answer to it at all, but merely respond with another greeting, or say the same thing myself, perhaps even simultaneously. Alternatively, I may also respond with something like:
"A eto..."
"A evo me..."
"A evo, jebiga..." [expletive! -- to be used carefully]
All of these are essentially meaningless expressions saying something like "Here I am." Other people have their own ways of replying, which I usually don't even notice.
Depending on the way it's said, this greeting can also express a warm feeling of having missed the person you're greeting, or a mild, friendly scolding for neglecting the contact. It is possible to reply with a strongly stressed
"Gdje si ti?" to return back a similar message.
Overall, this greeting is used only in very informal and friendly situations, and it's not even appropriate in all such situations (e.g. normally you shouldn't use it to greet a romantic partner

). Generally, if in doubt, don't use it!