Where is the leak coming from?

It maybe your main coolant return line, this comes off the bottom of the water pump and returns coolant back to your radiator. HOWEVER, if you ad coolant and it is still getting hot, you are most likely leaking from your overflow. This would be caused because your car is getting to hot, the coolant is expanding and spilling out your overflow (this would leak from roughly the same area). Your car maybe overheating for multiple reasons, but its likely one of two things, either your thermostat is stuck closed, not allowing proper coolant flow, or your electric radiator fan is not kicking on. To check the fan, when the car is running and warm, with the hood popped, look at the back side of your radiator, you should be able to hear, see, and feel the fan running. If that is working well, Your thermostat is the next place to check. This can be pulled out by removing the thermostat housing where the radiator hose attaches to the top of the motor (really simple, 2 bolts), take the housing off, pull out the little metal thing with a spring in the middle, put the housing back on and see if your car is running cooler. This should cause your car to run cold all the time, if that fixes your problem, buy a new thermostat (like $12) and pop it back in the same way you took the old one out and you should have your problem fixed.

Best of luck

P.S. Check your oil and make sure its not a milky color. Overheating a car can blow a head gasket and cause coolant to leak into your oil, if its driven to much with a bad head gasket the motor will blow up.

P.P.S MAKE SURE your car is cool before you try working on the cooling system. If not, boiling hot water will spew out and can cause serious injury.