Replace the thermostat gasket.
You will have to locate the source of the leak before you can fix it. Re-post with your findings.
You have to determine the source of the leak then decide on how to fix it.
I had an external antifreeze leak, and I tried a product called steelseal and it worked as advertised, just took off radiator hose and added the product as per instructions still no leak, i think it messed with my thermostat though.
depends where the leak is, and how bad it is.
antifreeze leaking into the oil pan
Find source and fix it.
Pressure test system to determine where leak orginates to determine fix
Have cooling system pressure tested to determine origination of leak
by getting a Bars Leakradiator leak at walmart
Start with replacing the thermostat.
try replacing the seal