Well now, ideally you would wear a suit. A suit consists of trousers and jacket (and sometimes waistcoat) made out of the same cloth. Get one in navy or charcoal grey. Black suits are frowned upon in these parts. A nice navy or grey one you'll also be able to wear to weddings, funerals, job interviews, court appearances and other important occasions.
However, it's only a prom and if you don't have to money for a new suit don't sweat it.