Assuming the Aux jack in your car is a standard headphone jack get a 3.5mm Stereo Headphone Cable to connect the iPod the the Aux jack. Set the iPod's volume to roughly 75-80% then set the volume on the stereo.
Another method would be for you to purchase a car accessory (make sure it works with your iPod) that plugs into your car's cigarette/12V adapter and has a line-out on it. That way your iPod is charged while you are using it in your car and you don't have to worry about adjusting your iPod's volume. I have an accessory like this in one of my cars and it provides superior results to just using the iPod's headphone port.
I suggest that you follow Sparkee's suggestion until you find the right car accessory for you.
It would be a waste to purchase a charging+FM unit now (those are rather expensive) just to hold them over until they purchase a charging+AUX unit. They would be better off using their iPod's battery until they can save up the ~$25 for the charger+AUX unit. My point is why spend over $50 in the long run when they can just spend half of that now (or in the near future) for the AUX accessory?
Even then it would be a waste of money. Why? Because I can pickup an auxiliary car charger for about $25 when most FM transmission+charger units are $40 or more. Why spend almost double the money for something that produces terrible quality? So I stand by what I said: it would be a waste of money to purchase an FM transmission unit. This is especially true since the iPod's headphone output port (directly hooked up to an aux input) will sound world's better than ANY FM transmitter (that is not a matter of opinion either, that is a fact).
Just use the iPod's headphone output port until an auxiliary+charger unit can be purchased. It makes no sense to use an FM transmitter when the OP has an auxiliary connection in their car.