You could answer "I'm alright" or just "alright" (alt. spelling: all right), and also "I'm good". That's what I can come up with right now on the top off my head. :)

The standard answer to 'what's up?' is 'nothing, how about you?'

Hi Ting, when a native speaker say 'whats up' it's just like he/she says 'what is the news?' you could answer by saying 'a lots' just to cheer up or 'fine, thanks'. Have a wonderful time.

thanks a lot

Instead of saying I'm fine you would say " I'm good" . For me if someone says what's up when talking in person or on the phone the first thing I'll say is " oh nothing much" .... And then anything afterwards say anything ( I'm doing well, I've been busy, I've been stresses out, etc" and then I'll ask " how about yourself? Or how are you doing or how have you been? But there's many ways you can respond what's up? Is basically asking "how is the status/conditions in your life " so you can reply based anything about how you are feeling