sti phone cards

Trying to buy a car…and trying to figure out how?

So im trying to buy a subaru impreza sti 06 and they run about $32,000. i have had some stuff to do with credit and all like credit cards and phone plans…my parents are now saying that if i join their american express stuff that it will definately drop interest prices on the car…im like in a position where all im getting is nothing but negative feedback like “no dealers will screw u over and this and that” i understand that…but no1 else is gonna sell the car but the dealer…so will that american express stuff work and how long will it take? or should i just go and try to get the car the old fashion way…just drop some money down and get it over with…thanx in advanced…