You've got several options, but first could you tell what kind of car stereo you have? Is it a CD player? does it have a USB input or is it one of those old tape decks?
1) if your car has a regular analog audio input (one of those regular cables with two headphone jacks at each end), you can plug one end of it into your phone and the other into your car stereo. It's the easiest way, but the audio quality isn't always perfect.
2) if you have a bluetooth stereo you can stream music via bluetooth between your phone and car stereo. this does drain your battery a lot though.
3)if your car has a USB port plug in your phone's USB cord into it and it should work
4) finally, if you have an old tape deck system, you can buy one of those ancient tape deck adapters and plug in the other side of the chord into the phone's headphone jack.
There are also FM transmitters that transmit your music on an empty radio wave length, but I wouldn't recommend them too highly.