"Are you a girl?"
you must have made some mistake. Possible mistakes: 1) it's spelled "czy" 2) czy does not really mean "are", or "do", or any other word you put at the beginning of English question. It is a yes/no question word, you add it at the beginning of the sentence, but you still need the verb.
so "czy jesteś dziewczynką?" or "czy ty jesteś dziewczynką?"
Do you mean pronunciation? Because there is no "jestesz."
Ś = /ɕ/
Sz = /ʂ/
The Ś is more palatalized than English "sh" /ʃ/, and the Polish "sz" is less palatalized than the English "sh." For the "sz" sound, the tongue is in the same position as English "r," but blows air like "sh"