Check to see if your car has an auxiliary plug in. The plug would be a small, round hole or port labeled "AUX." If not, make sure your car has a tape deck. If you do have an auxiliary plug or a tape deck, your guitar cannot be plugged into your car's sound system.
Purchase a basic auxiliary cord if your car has an auxiliary plug. If you have a tape deck instead, purchase the auxiliary cord with the tape deck adapter.
Attach the auxiliary cord to both the car and the guitar. Turn down the volume, bass and treble features of your radio significantly before playing a single note on your guitar. This will prevent damage to your car's speakers.