Most LED's are pretty bright with about 20 milliamps (that's .020 amps) flowing through them (check you data sheet for your specific LED). So.... 12v - 3.3v =8.7v 8.7v/.020= 435 ohms Now... you can light an LED with less current, you just have to experiment. It depends on the LED you are using. The lower the resistance the higher the current will be. Going with a standard 1k ohm resister will get you about 8.7 milliamps. The LED should work just fine with that. Good luck.