Why are you having your ball/prom in August?! Or are you just getting prepared ultra-early?!
Depends if it is a ball or a prom - there is a big difference! If it's a ball, it's black tie - so it has to be a tux. If it's a prom, it's less formal, so a nice suit will do, but some people do wear tuxes, so you'd need to check with your friends to see what they're wearing if that's the case.
With a tux, I think a waistcoat looks really good, less plain than just a shirt or a shirt with a cummerbund. I especially like the combo of a black tux, black shirt, cream/ivory waistcoat and cream/ivory bowtie! I'm not mad about black tuxes and white shirts with any other colour bowtie than black really - red can look ok, but sometimes people just wear random-coloured bowties that just don't go.