12v (ish) measured by a 0-3.3v input pin
A simple way of doing this would be to use a series zener diode.



Below 10V the zener will not conduct and the output will be 0V.

Above 10V the zener will pass current into the voltage divider. This divider reduces the voltage (0 -5V) to give a 0 -3V3 output. 

The .1uF across the 330R helps to filter noise.

The Zener current will be determined by the input voltage, zener voltage and the two resistors. In this case the maximum current will be 9.4 mA  ((15 - 10) / (330 + 200))