There is a seal in the valve cover that is leaking. You would need to remove the valve cover and replace the seal. You can buy a kit that does the spark plug tubes as well if those are leaking too. Biggest thing is to clean the block and valve cover where the heads, front timing cover, and valve cover meet and then use a glob of RTV sealant at that "T" junction when reinstalling the valve cover. Mount valve cover, bolts finger tight, let sit for 1 hour for RTV to flash over, then tighten bolts in correct sequence and torque spec. Let sit overnight to cure before starting.