How to measure high current (20A DC) using arduino
You can use an isolated or a high-side current sensor. LEM makes some very good ones (however they are not cheap) that are non-contact- just run a wire through the sensor. 

There are ICs that work from the high-side shunt and pass a current proportional to the sensed current. For example, the Si8540. You do have to ensure that there are no transients on the +12V line that exceed the 36V capability of the IC or it could be destroyed. There are a number of other similar chips available. 



Finally, if you have a low-side shunt, you can simply use an op-amp amplifier.