If it is coming from the frost plug as Gary suggests (and I have posted a photo below), you will have to drain your coolant and remove and replace the leaking frost plug. They are 22 mm or 7/8 inch (either will work) and can be found at any decent engine machine shop (or even NAPA or many auto supply stores). Most are galvanized steel, but Gil also sells a brass plug. I have always used the steel versions. Just drive the old plug in the middle with a punch to dent the inside in-wards. That loosens the plug, so then I usually drive a small chisel or punch though it and then pry it out. Clean the hole well to remove any scale or rust. Install the new plug by smearing a light coating of sealant around its outer edge and drive it in place using a socket just a bit smaller than the outer diameter. You want to hit it along the edge, not the centre, or you will loosen the plug as you install it.
HTH