Avoid products which claim to block any leak from the inside (Holts Radweld and similar). They can clog more than the leak itself, the heater matrix or the smaller passages inside the engine might suffer. Do it properly and thoroughly instead.
First of all, locate the leak. Look for the usual whitish residue around hoses and clips. If a hose is ruptured, replace it (on the roadside, you can use Holts Hoseweld to bandage a hose but replace it with a new one as soon as you can). If the heater matrix is leaking, replace or repair it.