You mainly need these things:
- The app, which you get from the iTunes App Store for $15 (http://itunes.apple.com/us/app/cms-lap-timer-pro-data-acquisition/id434692438?mt=8)
- A GPS module about $100
- A way to mount the iPhone in your car
The two best GPS modules are the Bad-Elf and Emprum Ultimate-GPS. The latter is currently the best choice because it supports a GPS position update rate of 5 times a second. All the other choices are currently only supporting 1 per second. However, the Bad-Elf should support unto 10 times a second very soon. My suggestion is to wait until close to Xmas before ordering a GPS module since the all cost about $100 and the Bad-Elf has the best potential.
The simplest mounting solution is a windshield mount. I like the ones available from Ram Mount. If you have a roll bar, then I like the Ram-Mount V-Mount.
More info is available here:
You also need an in-car power adaptor. I prefer the higher rated ones so it can also charge my iPad in the car. And, I prefer the ones that have a USB adaptor in them so, again, I can use it for charging other USB compatible devices.