Depends on where your at. Your in asia so I would say nothing much. Or im ok. Nothing much whats up with you? Usually people in us say whatsup and then the other says whatsup back. Also in the west people say im chillin when asked whatsup. meaning relaxing that is street slang though. haha. Anyways Say whatsup back is the best or say nothing much. Or tell them what your doing. I am going to the store going to the park to work etc.

When people say that to me I always say 'the roof' or 'the sky' haha A standard answer is 'not much, you?'. It's also an invitation to bring up whatever topic you feel like ex: "What's up?" "I'm just thinking about..."

Naturally, nothing much.

you can say . Nothing is up . nothing much

I use to answer 'all great!' :)