Given the resistors you have selected, you can use Ohms law to determine the current that will flow through the resistor, as retrolefty points out.

Then, with the current and voltage known, you can compute how much power the resistor is consuming (voltage * current = wattage). Make sure that the resistor you select has the appropriate wattage rating.

Your 51 ohm resistor at 12 volts (V=I*R) will draw 12/51 amps. That's about 200 mA.

200mA * 12 volts = 2.4 watts.

That's a BIG resistor. You won't find that at Radio Shack.