what you will commonly used to answer when someone asks you that "what's up"?

no thing just listening song

Hey! I'm good. You?

Not much bro is how I would say it in English.

i will say:fien ,and u?because i think what's up the same as "how r u?

 it's hard to answer , but i guess it well depend on the situation , always on the situation :) LOL!! 

   well, if i would answer it " im fine!!" hahah

not much, just livin

The following might seem a bit weird to you, but, this is a slight mix of how we talk naturally around where I live and well... "proper" English.

Nae/No' much at all really, wha' bout you/yersel'?

Nae/No much.

Nothin much.

Not a lot, you?

No' an awful lot.

or if you're asking it in a how are you manner

Aye, am no/nae too bad.

I'm good.

I'm fine.

and for those silly moments

The sky

The ceiling

The sun

fine,what about u?

I would say "Not much, you?" or "same old, you." "Same old" as in "same old thing."  

I would answer with how I feel. Like "I am fine/not fine, how are you?" :-).