Sweet 16 Birthday Party In August

I am having "My Sweet 16" and I don`t know what theme I want. I have 4 in mind so, should I go with the cinderella theme,hollywood theme, the car theme, or the casino theme?My favorite 2 themes is the casino theme and the cinderella theme. I was wondering if I should throw the party in my backyard since we have 9 acres and we could decorate and get a dance floor put out there? Should I just rent a hall or a part of the mall. I`m expecting at least 275 people. Then, I want thies party to be the best party I ever had. It`s co-ed and what should I wear and tell the people that are coming to wear formal clothing? I want to be looking like a million dollars. I`m excited about turning 16 but, I am also nervous.Please get to me A.S.A.P. So, I can make the reservations. The party`s in August.