That depends on the country.. in Australia it could go like this: A: Hi Aai, how's it going? B: Not bad, how about you? A: Not bad mate. At that point either person can start the real conversation. "How's it going?" isn't a real question, it's more a way of saying hello. Another option is "I'm fine thanks, how about you?". It's a bit more natural than "I am doing good". Even if you're not fine you would usually start out by saying you are, then talk about how you are really feeling later.

My usual reply is: "Same siht, different day."

If someone asked Hi Aai and How's it going? You can answer with your emotion and feeling if today you felt good then said fine, but if you don't you'll probably say bad or awful. In simple way, you should say : Hi ... I'm good, thanks and you?

i always said "everything is fine" :D