When someone says "Jazakallah Khair" to you, the proper response is "Wa iyyaka" for male, "Wa iyyaki" for female, and "Wa iyyakum" for plural. This simply means "and to you too." A more formal response is "Wa antum fa jazakumullahu khayran," which means "And you too, may Allah reward you with goodness". It is important to note that "Jazakallah" is an incomplete phrase and should be used with "Khair" to mean "May Allah reward you with goodness".