sup.....how've u been

commonly you say (I good) is ok

for what's up, you also can reply (Not Much), but not much cannot apply on how's greeting.

Due to how have you been is no different to how are you?, how do you do?, how are you going?

So you can reply:
I'm Fine, I'm bad, definitely good....you can reply from your actual feeling. this does not have to say everything good if you have something happened bad.

sup= what is up it's basically asking wt u hv been doing... usually people reply "nothing much" or they juz simply tell them wt they have been up to...
how've u been= how are u u can juz say "great (fine)"