Well, you can say whats up with you? or simply answer the question by saying "All good my brother", for example.

Well you dont need say anithing just Alright , Im good or Im fine. but if the other person said whats up my friend that mean he or she want still taking to you.

It is a very casual way to say 'hello", small talk like in an elevator or in passing briefly "Hey, what's up!" response: "Not much, you?" you may also see: "Hey, how's it going?" response: "Good, how about you?"

Thank you Nikky!

"What's up?" depends on context, but it is more casual. Among friends, this is what we use to greet each other, similarly to how you would use 吃饭了吗 in Chinese. We use “what's up” . In this case a common response would be: "Not much (you are not doing anything important). You?" "What's up?" can also be like 你最近怎么样 or 你在干嘛， where the person asking would like to know what you are doing or what you have been doing recently。