Outfit for my sweet 16?

for my birthday i am having about 20 people and we are going to eat at fuji japanese steakhouse, then a few of my closest friends are going to spend the night at my house. everyone is gonna wear casual clothing. i already know i am going to wear a tiara on my head but i dont know what i should wear( a dress, romper, shorts and shirt?) any suggestions on what to wear and where to get it? oh and my sweet 16 is in may