OK, don't forget that you cannot put 12v into a uC pin. You will need to lower your signal, probably by using a resistor divider. Take a look at the following image from Wikipedia:
**broken link removed**
You could use a resistance such as 10k/10k or 22k/22k for R1 and R2. This will halve your input voltage, so 12v becomes 6v, 5v becomes 2.5v etc. Still a little high for the microcontroller, but easy to double in software.
By "trigger point" I mean what voltage would you like the microcontroller to detect? e.g. if your input is 1v to 4.5v and you want it to come on at 3.25v, that is your "trigger". Not an official term, but I think it describes it.