You can make a [u]protection circuit[/u] with a resistor and a diode.
Or, you can make a [u]voltage divider[/u] with 2 resistors, but add a "backwards" diode between the Arduino input and 5V because you can get voltage spikes on an automotive (or tractor) electrical system. The resistor values are not too critical... It's the ratio that's important. The resistors can add-up to about 10K. Higher values are more susceptible to noise and lower values will draw more current (and overheat if they are too-too low).
valves are operated via 12 volts, basically straight from the battery, they are either on or off (not proportional)...
...The joystick gets 12 volts directly from a plug in the tractor (basically same as in cars, which you use for charging your phone), the potentiometers from the axis send out 0 to 2,4 volt and 2,4 to 5 volt (forward/backwards or left/right on the joystick),
It sounds like if you can find the "right" joystick with switches instead of pots, you wouldn't need the Arduino.