What material of sari will make me look slimmer?

i'm a 18 yr old....n i have to wear sari next month for a school function.....i'm 33-32-34...n 5"1 .what material of sari will make me look slimmer?this is the first time i'm wearing a sari so quite nervous too..i dont have a flat stomach...should i tie the sari below or above the navel?