Little bit of planning first.
Grab some paper and list what you want to monitor down the page then add some columns.
• Is this an on/off.
• Does it go high or low for the on.
• Is the voltage fixed or variable. Eg: will it go to 12 volts like the lights going on or is it going to be variable like battery voltage (12.7 volt at rest, 13.8 to 14.4 while the alternator charges).
• Is it a voltage or a set of pulses like some speedo's and rev counters.
Like programming you cannot set up the program without knowing the parameters.
With the 12 volts you CAN use a regulator but this is excessive as you have to have the regulator and caps to stabilise them an there will be a lag as the regulator comes up and stabilises. This will cause you to miss some short pulses.
The quickest way is to put a voltage divider to give you 5 volts or more into an inline resistor that goes to a zenar diode that will start to conduct. Have a look here.