“Good you” is the perfect response, its just a more personal version of Hi or Hello, no need to over think it. As for the second part if you know the person hows it going can just be a conversation starter, its meant to ask what are you up to, i.e. is there a light topic we can have small talk about that isnt going to be too involved. You can respond with something along the lines of “I’m doing good, such and such happened the other day that was nice, how about you.”