Coolent

Where is the leak coming from? If it is leaking from a radiator hose, either tighten the clamp or replace the hose. Same thing with heater hose. If the radiator is leaking, you can take it off, take it to a radiator shop, see if they can fix it? If it is a gasket leaking, have the gasket replaced.

Your local autostore may have some stopleak that you can pour in the radiator to stop leaks. I won't use that stuff except in an emergency.