Behead the fish. Now you could do this right at the get-go of the filleting process, but then you don't have anything to hold the fish down with when making that first cut. Those who start with the tail often remove the head and guts first.
This is where you use the cleaver. Chop the head off righ where you cut that first fillet.
If you are going to make stock, now is the time to cut the gills out of the head. Gills are the other thing that is best thrown out -- they will make your stock bitter if you leave them in. Cut the gills out with the fillet knife; they are attached at either end by strong ligaments, but only by a thin membrane in the middle.