Use a voltage divider to lower the voltage so the maximum is less than 5. Input that to one of the Analog input pins (not the AREF).
You can try a 3.3K and 10K resistors wired like this:
12V ------ 10K ------ Analog in ------- 3.3k ------- GRND
You can calculate your voltage like this:
voltage = (analog in) * 5/1024 * (10+3.3) / 3.3
or
voltage = (analog in) * 0.01966
Don't forget to use floats when you have decimals.