Its sort of a rhetorical question. Its just a greeting that friends sometimes use. You can say: 'hey man', hey there', 'not mucht', 'nothing', etc.

You can either answer with something like "Hey" or you can respond with the same thing "What's up"...either one is fine. As for creepy, we don't use that word a lot, unless if it is to describe something that is weird or odd or possibly scary.

Hello Ahmed. Your understanding of "What's up?" is correct. "Creepy" describes unpleasant feelings. For example, if you were visiting a distant city, and went walking at night, and you began to feel unwelcome, because people stared at you in a strange way, you would feel "creepy". The place would be creepy, and the people would be creepy. It isn't necessarily something specific, but just an uncomfortable feeling that suggests to you that you are not really safe in that place. By the way, I have a neighbor who is "creepy". He knocked on my door one time, to ask if he could borrow some tool. I was immediately struck by a wave of unpleasant feelings. I have no explanation for it, but I felt as though I was talking to a man who did something like torture animals or tortured small children or something like that. I have never made the man feel welcome, and I always have the strangest feelings from this fellow. There is nothing cheery or positive about the impression he makes. That is a fairly good description of "creepy". .

"What's up" is, for the most part, an impersonal and an informal way to say "hello". It is asked to both guys and gals. Probably the most common response is, "not much." It seems to me, that unless you are really good friends, then "not much" is your best response. If a good friend or family member is asking, then it could mean, "hay I heard something today about you, what is going on?" or "what are you doing today?"