Student or Learner
What does "What's up?" really mean?
Somebody says it means that "How are you?" some says "What are you doing?". I am very confused. Can you clarify this please?
What about in AmE?
As far as I know, in AmE it just means "How are you doing/How are you?" as you suggested. It's a fairly common expression, especially in conversation.
Please be aware that I'm neither a native English speaker nor a teacher.
It's a greeting, basically. It's not a serious inquiry about anything. One would answer with a "not much" or even a "what's up?" in return.
This is casual/slang, of course.
In AusE, it can mean "What's wrong?"; "What's happening?"; "What's new? (Is there any news?)" - depending, as always, on the context.
It's not used on its own as a greeting.
I am sure it might have another meaning when it comes to Canadian English.