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If you want to calculate the value of the necessary resistor, you'll need to know the voltage and current of the LED you're using.
Then you can use this calculation to determine which resistor you need.
R = [V (Battery) ➖ V (LED)] ➗ I (LED)
So, for a typical standard LED with a forward voltage of 2V and a current of 20mA, the necessary resistor would be:
[12V ➖ 2V] ➗ 0.02mA = 500 Ohm
An extra bright LED may have a forward voltage of around 3.2V so the required resistor for that would be:
[12V ➖ 3.2V] ➗ 0.02mA = 440 Ohm
The resistor should be wired in series and is not directional so it doesn't matter which way round you install it.
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