The cheapest and easiest way to fix a vehicle's radiator leak is to add a bottle of cooling system sealer to the radiator. However, this is only a temporary fix; you must eventually employ a more permanent fix, or have the radiator replaced, depending on its material.
After purchasing the sealant, locate the radiator cap. Making sure that no coolant leaks out, pour the recommended amount of sealant into the radiator. Alternatively, if you have no sealant, you may pour 2 tablespoons of ground black pepper into the radiator instead to attempt to from a temporary seal. Add radiator fluid into the radiator to return the coolant level to the manufacturer's specifications, then replace the radiator cap and start your vehicle. Allow your vehicle to run for five to 10 minutes. After this, remove the radiator cap again and fill it with a mixture of one-half water and one-half coolant. Allow the vehicle to sit overnight.
If your vehicle's radiator is copper or brass, you may attempt to solder any leaks that may be present. Many newer radiators are aluminum, and these can be repaired with an epoxy glue if any cracks or pinholes are present. If the radiator is plastic, you need to replace it if it is leaking.