Get one of the DC to DC convertors listed in the store if you are going to be running a number of things off of it. If you look I dare to bet your PI has a 5V DC input. All of the small arm boards I have looked at do.
This at a minimum would require a simple 12v to 5v dc adapter that could plug into the cigarette lighter to power your device. If you are going to permanent mount it you may want to consider a version you could hard wire into your car. Many have the hardware in the plug that goes into the lighter socket. You want one with the hardware somewhere else so you can cut off the plug.
The adapter or "wall wart" that came with your pi is likely an AC-DC adapter and will say the voltage it puts out and the amperage. Pay attention to this when you get a DC-DC adapter. A screen, unless you run it directly off the PI, will likely come with its own lighter type plug but the screens I have seen are made for 12volt systems so you couldn't run both off the same adapter without some additional hardware. But check your system for details.