Trying to make a usb car charger
As noted by retro - you can buy converters that will probably do what you want from Amazon or ebay or similar for a few dollars. 

And you could use a linear regulator (with series input resistor as per my comment) and a heat sink. But, if you WANT to make your own and learn quite a lot and hav something that will be useful for many other tasks, then an MC34063 is a cheap and easy way to do it.

Agh!!! - just had a look at my answer of May 2nd.   Just do this :-}

Ancient but glorious MC34063 + big brother app note

Claims to assist in component selection  

Another version  fromhere but its a stolen 2006 Silicon Chip circuit. Bigger version here 

For inductor they say 75 turns of enamelled 0.5mm dia wire on Neosid 17-732-22 powdered iron core.



The diagram and notes below are from figure 11, page 7 of the MC34063 data sheet from OnSemi.
 Digikey $US0.48/1 for SOIC-8 and $US0.62/1 for DIP8.
 You also need:
 A 1N5819 1A Schottky diode $US0.40 !!!!/1  but far cheaper in modest quantity.
 A few R's and C's.
 An inductor. 220 uH 1A say 

In the circuit below L = 220 uH and Vin = 15-25V. Iout = 500 mA at 5V. It will operate below 15 V in  - a  smaller inductor would be better. Say 100 uH. The circuit is very forgiving of inductor value.