An appropriate answer to "What's up?" is "Nothing much." An appropriate answer to "How are you?" is "Fine, thanks, and you?" Yes, in VERY informal and quick exchanges with no further conversation expected, you can answer "What's up?" with "What's up?" In such situations in the USA, it is often pronounced " 'Sup? " and usually said with the head tilting backward once while saying "'Sup?"

What's up is an informal way of greeting someone or asking someone if they have a problem. A: What's up? Your reply: Not much. As a side discussion, another way to greet someone or ask for an update about their life would be: "How's life?" A: Hey buddy, how's life? Your answers: Fine, thanks. / Couldn't be better. *If someone asks "How are you", it would be odd to reply "How are you" immediately. You would risk sounding like a parrot, or come across as making fun of your friend's question. It would be better to reply "Great, thank you. By the way...... (continue with what you want to discuss)...."

It's not uncommon for someone to reply "What's up?" as a greeting without answering the question, but can sometimes be awkward if the person was genuinely looking for an answer, so that's up to context I guess. "How are you?" generally needs some kind of answer first, like "I'm good, how are you?" Even if the question is only superficial and not really asking for details about your life. Hope that helps.

No, you should answer the question first. What's up? = What's happening (with you)? The correct response is: Not much, how about you? You can also respond with a longer answer about what is happening in your life today.

"What's up?" can depend on context, but it is usually used as a causal greeting. In this case, we use "what's up?" similarly to how you would use "吃饭了吗", most commonly the response would be: "Not much (you are not doing anything important). And you?", or anything along the lines, but we typically don't just say "what's up" back. It is also often used as "你最近怎么样“ or “你在干嘛” where the person would like to know what you are doing, or what you have been doing recently.