LEDs drop a spesific voltage across them but this voltage drop does not set the current to flow through them (ususally), you need to pick a resistor that will take into account these voltage drops and then sets the current of the circuit.
The typical forward bias for an LED is between 1 and 3 volts, so if you have a resitor in series with this, it will be dropping 9 volts across I don't have the foggiest what you want to be driving your LEDs at, that varies on the specification, but you can use Ohms Law to decide on a suitable resistor value.
If you wanted to be fancy you could set up a current control mirror to drive your LEDs without dropping a load of voltage across lots of resistors, but then you're dropping it against a transistor instead.
One can use this simple equation stolen from wikipedia, but it won't be a perticularly efficent setup if you use that directly.
You would be better providing me slightly more detail and the spesific LEDs and I can work something out for you, or, (not to sound horrid) but think of how you can set up an array of LEDs with rows of them in series to drop about 8 or 9V then drop the remaining 3 or 4 across an appropriate resistor, and run sets of LEDs in series like this.
PS: Typical LED currents vary between 5mA and 50mA, (Power LEDs can run up to about 320mA)
http://en.wikipedia.org/wiki/LED_circuit <<This page may be helpful.
This page will be MUCH more helpful...
http://led.linear1.org/led.wiz
Select your source voltage appropriately (12V) the diode forward voltage should be listed in the datasheet, pick a current you're comfortable with, 20mA should be fine, but have a look at the datasheet for your LED and pick somewhere in the flat reigon just before it curves up on the graph (where a big increase in current only warrants a small change in light / voltage, that is your saturation current, avoid this area), select the number of LEDs in the array and ask to view the output as a wiring diagram, very helpful little page.
NOTE: Changing the voltage on your source (if possible) will allow dimming with a resistor controlled circuit.