I'm going to have a bath
Where does it hurt?
my head itches, my head/hand is itching
I have an itch
this sweater is very itchy
he was itching to tell her
I got bitten by mosquitoes
Stop whining on about it!
Stop your whining, Billy. You can't have everything you want.
He always whines when his mother gives him chores to do.
If you keep whining, I'll take you out first.
I'm not sure about British English, but in American English you would say "I'm going to take a bath" and "Stop whining about it!"