What do adapter do I buy for an AUX port?

so i just recently bought a new car and i'm a bit car illiterate. i want to connect my ipod to the car stereo so i can jam to it, but there isn't a cord that's already connected. i know there's an 'aux' jack, but i'm not sure what adapter to buy. is it the double headphone jack thing, or is there a specific one with a headphone jack on one end and an 'aux' jack on the other?