Will a black cocktail dress look good on me?!?

i want to buy a black cocktail dress (short). I'm pale, have red hair and its like shoulder lenght. I'm 5'9 tall and I weigh 170 lbs.. I don't look fat. One more thing, how can I perfect my porsture??! I always walk like hunchback, but its not like Waaaaay weird xD its just a liittle weird.. But i look better standing with a straight back. I ALWAYS forget to walk/sit straight.. Any suggestions on how to remember myself to walk/sit straight?! Thnx!!!!(: