This is a voltage divider that will give me a max of 5v out of a 12.6v (max) input. Of course this is linear, and at the moment this will draw 3.8×10^-6. Hopefully that will be enough for the ADC to pick it up, which I am very skeptical about being very close to 0.
Arduav 
How do I use a voltage regulator like the LM317 or a LM7805 or a LM7812 ?
I think the 78xx series will give about 1-2 amps.
What current could I expect from the LM317 pushing out 5v ?
Datasheet for the LM317, http://www.fairchildsemi.com/ds/LM/LM317.pdf
states the typical current draw for less than 15v is 2.2A.
At 40v it’s about 0.6v.
The others are rated for 1A.
The 7805 is a DC-DC voltage converter which will always output 5V regardless of input.
Get the TO220 with the metal back to connect to a heatsink if need be..
http://www.learningaboutelectronics.com/Articles/How-to-connect-a-voltage-regulator-in-a-circuit
http://dawn.singlespoon.org.au/build4.shtml