AS reds, HP/level yellows, MR/level blues, ArPen quints or other...
There are a lot of build for Skarner, some like to rush Trinity Force, I prefer to go tanky.
I start with HP crystal, boots and tome, then I get merc treads and sheen.
after that, depending on the enemy team, I get glacial shroud or wit's end first... but I'll get both.
If I need more armor, I upgrade to frozen heart. If I need more MR, I get odyn's veil.
as last item, revolver to gunblade.
with sheen and wit's end, you'll deal enough damage.