Why nya and not ny?
It's because nya is the definitive form of the adjective in min nya klänning, not the indefinitive form as in en ny klänning.
As Zmrzlina recently said, "Adjectives describing something 'owned' by another are treated as were they definite."
isn't ni a a plural form of you?
Yes. It's also used as a polite form in some areas (but considered impolite in other areas! Safest not to use it) . Duolingo version is picking a random pronoun for you to translate.
MarkBorkBorkBork, help me understand your response to OscarMPK. Are you saying that in some areas is impolite to say, "Vill ni titta på min nya klänning?"