R = (Vs - Vf)/if is the formula to use!!
In english this is, V is voltage supply(use 13.5 volt) Vf is the forward voltage drop across the LED, If is the forward current through the led.
You should get this info with the led when you buy them.
EG,
LED if = 20mA and Vf = 2.5V,
To operate from a 12V DC supply R =(12-2.5) /0.02= 475ohm anywhere within 10% resistor value will be fine, so that is 12-2.5 = 9.5/0.02(of an amp) 475 ohm.
In short unless you know the forward voltage drop and forward current you cannot work it out only guess, but most led around 20-60ma, if you err on the side of caution it will still work, just not as brightly.
It looks a lot harder than it is, I would suggest Maplin or similar for led's to ensure you get this data, I use this all the time for overpowering and sinking CREE led's(bright white ones for torches).
tabetha