When someone asks 'what are u doing?' this question doesn't require a specific answer. You could answer saying, "Nothing much" or "Just the usual" because the person likely doesn't need to know the ins and outs of your routine, it's just a conversation starter.

1 :when someone ask"what are u doing" and I am no special things to do at that time(I guest he want me to help him to do something and i would like to help him) how to answer he ?[ I'm doing nothing at the moment. Sure I'm free." 2: my friend drive me to buy some items, when he ask "do you have any special items to buy?" how to answer if I just want to buy some everyday life items .[ Just say, "I don't need to get anything special, I just need bread, jam and dishwashing liquid etc"

1. (I'm doing) Nothing particular. 2. Nothing particular. Actually both answers are the same. It's a very useful expression. Learn it.

That must be when you are online ,this question is not asked in real life conversations or small talks,because the person online is just trying to open a conversation or break ice,but it is a silly line to use. They probably have nothing to say and just want to find a way to start a conversation. It is a thoughtless way to initiate a chat. Just tell them I am chatting with you,what you will be really doing instantly.