This is my first build and it will be very short. All Items, masteries and runes are very situational, depending on opponents and style of your play. This guide is not meant to learn you how to play Yorick, its purpose is ONLY to introduce interesting possibility to play Uncle Gravedigger, so don't comment that it is incomplete - it is, on purpose.
Please, post your ideas in comments, thanks :-)

Cons

• Not so strong early game
• has to farm a lot
• slow

Pros

• High damage output
• Almost a tank
• Can kill most of chars 1v1
• Will not die easily
• Can slow enemy
• Can adapt to almost any scenario

Runes depend on every individual, but I would generally recommend Lifesteal quintessences and normal runes for AD, life, armor penetration or cooldown. To achieve best results, play several games, find out which items you like the most, and then select runes to benefit your playstyle.

This is purely a matter of personal preference. If you expect easier lane, start with doran's blades and buy boots later. If you expect to get beaten, start with boots of speed and potions, followed by dorans. Against AD-heavy team, doran's shield is also a viable option.

In this phase of the game, Yorick is quite weak. Focus on farming and getting Trinity together with Guardian Angel ASAP. After that, you will shine and farm form first Bloodthirster quite easily.

In lategame, items are purely situational.

• Are you fighting okay, everything nice and smooth? Go for another bloodthirster, infinity edge or phantom dancer.
• Do you have hard time surviving in teamfights? Buy Warmog
• Do you receive a lot of Magic damage? Buy Force of Nature, Maw of Malmortius, etc..
• Do you receive a lot of Attack damage? Go for Thornmail or Randuin's omen

• Q: I am so damn slow!
  A: Yes, however, if you use ghost and your items properly, communicate well with your teammates, this should not be such a big problem... And you still have that exhaust, remember?
• Q:Enemy AD Carry is ripping me appart!
  A: Just use exhaust on him, your Q and show him who's the boss.
• Q: Where is any armor penetration?
  A: Most of the times, you won't need it. You are offtank, your aim is to kill AD carrys and casters fast. You can chew tanks later... But Feel free to buy any armor penetration item in lategame...
• Q: I don't have any more questions!
  A: Right.