you have 3 options
1) FM transmitter, ex. iTrip
2) your stereo might be able to get an iPod adapter (Alpine, Pioneer and i think Kenwood all have this, it's like $100 and you have a dock connector cable that plugs into your ipod and you control it from you stereo) and you can get kits that work with certian cars, like the VW and Honda stock stereos have a thing you can add to them.
3) Line-in, some stereos have a 3.5" jack on them, you get a 3.5" male-male adaptor and you go from the headphone jack on the iPod to the car stereo, my 93 VW stock stereo has this and it's what i use, but most cars dont have this.