well,if you are in a good mood,then just say "fine"or "quite good!"
if you don't know what's proper to say,then "soso"may be a good choice,however most foreigners don't like Soso,they'd prefer "all right"or sth else.
if you feel terrible,just say"bad" "terrible" with a sigh.
Actually ,you don't need to take this question seriously,it's just used to start a conversation ,so,never mind.