English question, please help.?

Both "What's up" and "Wassup" are slangs/colloquialisms, but "What's up" is far more commonly used. "Wassup" is very rare.

"My hand is itchy" & "My body is itchy everywhere" are both acceptable, though it is probably a little more common to say "My hand itches" or "My body itches everywhere" or "My body itches all over".

I'd say "my hand is itching", but maybe that's just me.

And for the "what's up"-question, I change it everyday. Why stick to one version? My dad's 45, he says everything from "wassap" to "What's going on with you?". Variation is nice.

We would say "may hand itches"
Most people say "what's up?".

Pretty much how you've expressed it.

As for "what's up" vs. "wassup," it really depends on who's saying it. A white person could sound like a square saying the former, and like a square desperate to be cool saying the latter. Lose/lose either way.

Most people would say whats up mate unless they are teenagers they would say wassup x