There are different ways depending on what kind of inputs your car stereo has. The most universal solution would be an FM radio adapter. These connect to your device and send out a signal which you can tune your car's radio to. The only problem is that in cities with a lot of radio stations you may not be able to find a usable frequency.
Alternatively if your car stereo has an aux. in then all you need is a 3.5mm male to 3.5mm male audio cable, with one end going into your touch and the other into the car. Or if you car still has a tape deck then you could use a cassette adapter. Finally if your car has an iPod adapter then you can use that with the touch.
if your stereo has a cassete player, then you could buy a cassette tape with a headphone jack built in. plug it in to your itouch, then put in teh cassette and it shouldplay.
YOu could see if you are has an auxillary port. Then get a auxillary cable (maybe 5-10$) and plug it in.
There are also these fm transmitters that you plug into your cigarette lighter, then attach your itouch to it via usb cable. It will broadcast a certain signal, and then you just go to your car radio and put it at the appropraite station.
Best solution would be some sort of iPod/iPhone car dock. Go to a tech shop like Dick Smiths or Noel Leemings, they'll have that sort of thing, and the guys there will also be able to help out find the right one for you and how to connect it. As you haven't given many details that would be the best option.