How can I determine where my Opel Corsa Lite + 2004 model is leaking water?

My Opel Corsa Lite Plus 2004 model is leaking water.
I noticed a water patch on the garage floor and checked the water bottle. After I have used the car I notice the water drops to approx 1 - 2 inches below the Kould mark. I continually fill before using.
What must I check?