Hi,
Park you car on a hill so that the front of the car is higher than the back. Remove the cap on the overflow cap and add water/antifreeze. When the engine comes up to temp check to see if the water/antifreeze in the tank goes down. If it does keep adding until you have a quarter full in the tank. If you see air bubbles coming out that would be great. Check around the water pump and hoses and the radiator for leaks. Check the wheel on the water pump and make sure it is working right,not wobbling.

If you want to ensure that you do not have a cracked block ( doesn,t sound as if you do) ensure the over flow is at a good level and let the engine set over night. If you do have a cracked block you will see a flim of oil in the overfow tank. There could be many more reasons for this problem but at least its a start.