Replace the thermostat gasket.
if the leak is at the thermostat housing, make sure you installed the gasket and that is torque down to specs.(you may try some silicone,not too much)
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.
You will need to replasce the heater core.
depends where the leak is, and how bad it is.
antifreeze leaking into the oil pan
Pressure test system to determine where leak orginates to determine fix
Find source and fix it.
Have cooling system pressure tested to determine origination of leak
by getting a Bars Leakradiator leak at walmart
Depends entirely where the leak is coming from.
An automotive product called "Bar's Leaks" will effectively seal any "small" coolant leak in the radiator but if the leak is coming from the engine area; this may require new freeze plugs and/or block sealer if the leak is not too serious.