Formal dress?

I'm 15 and need to find a dress for a formal buisness banquet. I wear a size 10, have brown hair and brown eyes, and look good in deep colors like dark brown, black, red, and deep blues and greens. Please try to keep the neckline high and appropriate for the occasion. The skirt should also reach to the knees at least, but I also like dresses that are floor length or ankle length - or are those called gowns? Please provide links. Thank you!