How do you form a question asking what is your nationality in French?

To ask what is someone's nationality in French, you can say: "de quelle nationalité es-tu (you singular and familiar) / êtes-vous (you formal or plural)" or "de quel pays es-tu" (less formal, would be more used in conversation).

To answer the question, say "je suis +nationality", or "je viens de +country".