Ouch! If it's not leaking out on the ground from something like a pinhole in the radiator or from a loose hose clamp then maybe it is going out through the radiator cap. Hopefully this will be all it is. If your cap looks like it's seen better days and the thermostat isn't working on the cap then you can just replace the cap.
If that isn't it then it could be a cracked head or a bad head gasket.
When you check your oil is there any milky looking stuff on the dipstick? If there is even the slightest then you either need a new head gasket or you have a cracked head.
Or...when you have your car running after a little while go back to the exhaust and see if there is any water coming out. If there is then you have a cracked head or need a head gasket.
OH, one last thing to check. Could the water be coming from your heater? Look on the floor of the passenger side and see of there is a bunch of water there. Or look under the hood on the inside of the fire wall where the heater core is and start your car, let it run and see if there are any leeks. But be sure to turn the heater on all the way up when you are checking for these kinds of leaks. If this is where the leak is coming from then it could just be a loose hose clamp or you may need a new heater core.
Good luck no matter what. Hope I didn't miss anything...
