"Not much. You?" If you don't have much going on.

"Not much, just finishing up this essay before I head to soccer." If you are doing something that might interest the person asking, or to explain your presence somewhere you aren't usually.

The person is just checking in with you. It's basically "hello"

This one is a bit nuanced. "What's up" can be used both as a greeting and a question depending on where in the conversation it appears. If it's a greeting a reply is not expected but rather a greeting. Here's an example.

1. A: What's up?
2. B: How's it going?
3. A: All good (note this is void of information, things are not necessarily good.

Another

1. A: Hi
2. B: Hey
3. A: Soooo, what's up? - Here a proper reply to the question is expected, optional to reply with "Not much, and you?" to skip the question. Normally people that ask in this way want to be asked this question in return.

Annoyingly, it depends on the context which makes it weirdly complex for a simple interaction.

I usually say "not much"

I look up, then answer the question. Usually by saying, "the ceiling".

Just reply back "hey". No one is looking for a real reply.

The normal response is, "Not much"

It is a prompt meaning "if you have something you want to bring up, or that is important for me to know interacting with you, now is a good time. I have made a space in the conversation for exactly that and I am listening".

So basically "nothing special" or "not much" is the correct response unless you actually are waiting to bring something up.

Continue with "...and you?" or "how about you?" is polite and invites them to also share anything relevant without worrying about "intruding" on your thoughts.

Caveat: Of course some people will say it as an empty greeting and not listen to a response. They are doing it wrong. If you respond with something and they are confused by that, it's them being socially inept and not you.

I usually hit em with the old

WE MEET AGAIN

"Same old, how are you?"

"A vector directly opposing gravity but greater in magnitude," if you want to passive aggressively tell someone to fuck off.

CHICKEN BUTT!!!

^(sorry I had to)^

"what's up!"

It's like a ping pong.

“My anxiety level.”
Or
“nothing, and you?”

"Livin' the dream"

"No, Signal"

"Hey" is shockingly acceptable.

"Not much, you?" is considered polite.

"My cholesterol" or "my blood pressure" followed by "my doctor says it's really a problem!" is my favorite response. People either chuckle at the joke and move on or get uncomfortable and leave me alone.

"Same old. The horror persists but so do I."

My flatmate says this whenever he comes in. I think he's just saying hello, but I always end up explaining in detail whatever nerdy shit I'm doing at that moment.

"The direction opposing local gravity."

Or "That way." While indicating with your index finger.

Usually gets a chuckle.

I colloquially prefer “notta whole lotta.”

Just chillin

Livin' the dream

"Chillin."

This is one of those frustrating social conventions that's partly a greeting and partly a way to find out if you're busy (particularly if they are asking if you're in the middle of doing something or are speaking to the other person on the phone/via message where they can't see what you're doing).

A good response, as mentioned in another reply here, is "Not much. You?" or "Not much - how about yourself?" if you haven't done anything out of the ordinary or aren't really busy.

If you have done something that you think the other person might find interesting, though, give them a brief answer, e.g. "Oh, just had lunch at a new restaurant. The food was great." Keeping it short then allows the other person to either ask a follow-up question if they are actually interested, or to move the conversation along to the actual topic they wanted to discuss with you.

If you are actually busy and in the middle of a task, then a short and polite response and suggestion to reschedule works, e.g. "Sorry, just in the middle of painting the spare room... can I get back to you tonight?"

Nuffin' much.

Alternatively,

Chickenbutt

There's a ton of nuance here depending on how it's said. "What's up" can be said so fast as a greeting that another "What's up?" is an appropriate response, or my favorite "sup?" Or, it can be asked kind of carefully that it expects an actual answer. Either way, no response is really wrong, but can increase awkwardness if not answering in the expected way.

It depends, but rarely the truth. It only took me 30+ years buy I finally learned that 99.99999% of people asking don't care. And they really don't want the truth if it's worse than 'so-so'.

My old boss told me that his son said the correct response is "how's it going, dude?"

What do you mean? They obviously want to hear a summary of what you've been up to since you last met.