Where exactly?
Ohv, ohc or flat head?
It could be a number of things.
Oil level, valves, gaskets, rings, bore. Maybe more.
Does it have blue smoke?
It shouldnt leak from the head, that's what the gasket is for. Pics?
If it is from the rocker cover, then try a new gasket.
The rings should have a clearance of a few thou. (.005" or so)
Have you got a feeler gauge?