This build is just what I use for itemization, If you want fancy explanations of everything Mordekaiser there are plenty of other guides out there that can offer that, my personal favorite is:
http://www.mobafire.com/league-of-legends/build/unforgivens-guide-how-to-build-a-successful-mordekaiser-24061

Marks: Magic Penetration (Standard)
Seals: Armor (For those pesky AD, and for early game survivability)
Glyphs: Magic resist (Survivability)
Quints: AP (Early game shield generation and poke)

Standard

Start off with an Amplifying Tome to make into a quick Hextech Revolver, this will significantly decrease the cost of Siphoning Strike when used effectively.

Final Items are (in order of build):
~Boots
~Rabadon's Deathcap (spell vamp help)
~Lichbane (Mace of spades dominance)
~Guardian Angel (Its funny w/ Children of the Grave)
~Hextech gunblade or Will of the Anchients (depending how much gold you have and how many AP users are on your team)
~Void Staff or Abyssal Scepter (depending on how many AP users are on your team)

Standard

Mordekaiser is still a good pub stomper, just can't be too aggressive in the beginning. Enjoy playing :]