EDIT: Beaten by lauralie2 - agree with her/his answer.
If you mean "How's it going?" when used as an informal greeting, you can respond in many ways. I would usually say "Not bad, you?" but you could just say "Hi" or "Fine thank you. And you?" or any other reply like that.
But "How is it going?" can be a real question if you are working on something. If so, tell the other person how your work is going. Are you having trouble or doing well?