First, you need to find the leak. Fill the radiator w/water and look under the car. This should help pinpoint the location. If you don't see any coolant leaking, there is only two other places it can go; into the combustion chamber then out the exhaust, or in to the engine oil pan. Pull the dipstick and check for a "milky" color and/or high oil level. If the oil is ok-start the engine and see if you have white smoke or vapor coming out the tailpipe.
