There's a couple of ways to do it, depending upon your head unit.
If you have a modern head unit that has the RCA inputs, you can get a $10 cable for your Ipod and you are good to go.
If you have a very modern head unit, it may very well have an Ipod interface built in (like the one I have).
If all else fails, you can get the FM transmitter for your Ipod that plugs into your 12v receptacle (or runs on batteries) and will allow to tune the radio to an unused FM station and listen to your Ipod music. I did this for the past year and it works great.
The direct cable connection is very, very nice and gives the best sound quality.