Welcome fellow grave diggers! I have seen a lot of concern about Yorick and his underwhelming damage output/ base stats. I've been trying out a few things and I have found them to work quite well and I'm hoping that they will work out for you too. Dig a little deeper and bury your nose in this guide (so many puns).

Pro's

You're a grave digger. Nuff' said.
Yorick is very fun to play.
You can run champs off with your minions and they tend to farm decently.
You can push towers fairly quickly with an Ult/Pestilence combo.
Your skills are named after the Horsemen of the Apocalypse- I say that's a Win.

Con's

Ranged carries such as Caitlyn will be annoying. Brush is your friend.
Yorick is currently UP according to anyone you ask. Buffs incoming per Phreak.
People may que dodge is you choose to play him.
You do not have the Hookshot in your skill set. What a shame.

SIDE NOTE* Whatever God/Deity/thing that forced Yorick to fulfill his 'quota' before allowing him to rest must be some sort of Troll God, seeing how Yorick is currently UP at the moment and has more trouble dealing damage than he should.

Strange runes eh? Kinda-sorta. Yorick's abilites scale with attack damage, two of them with bonus damage. That's any damage you add to your base damage. In order to get your WW to do an additional 22 and your E to do another 27 at lvl 1, we take all attack damage quints and marks (and a Doran's Blade/Long Sword). Not per lvl- those will not help you early on.
So, 9 Greater Mark of Attack Damage and 3 Greater Quintessence of Attack Damage. Seals are going to be Greater Seal of Armor because your base armor is atrocious.
Glyphs are magic resist- Greater Glyph of Magic Resist & Greater Glyph of Scaling Magic Resist for both early game and late game MR.
You might notice that there are no mana runes in here- that's because you will actually have to be judicial with your mana in lane. I haven't had to much of a oom problem in lane. It gets better after you have Tear.

I find this set up to be the best in my opinion. You don't get to benefit from Tenacity or Havoc unfortunately, but building 21 points in either tree kinda gimps you. :\ As a DPS, I take Deadliness , Cripple (for exhaust), Sorcery and Archaic Knowledge .
Why AK? Because two of your three damage abilities deal magic damage, that's why. Two more points into Brute Force to finish off your offense tree.
For defense, take three points in both Hardiness and Resistance , four in Evasion and one in Nimbleness . One more point goes into Hardened Skin and your last four go into Veteran's Scars . More health starting out is always nice.

I like to grab a Doran's Blade starting out for the extra damage it provides. A little lifesteal, health and bonus damage for War and Pestilence is what you need to get started. If you want, you can grab a Sapphire Crystal / Long Sword and two health pots. Taking the mana path, you'll be able to spam your abilities more often in lane and will lead to a quicker Tear of the Goddess. Both items will build into your eventual Manamune.
After that, craft some Boots and build them into some Ninja Tabi when you got the gold. Ninja Tabi and your masteries will help negate some of that nasty Physical Damage. I want to stress the fact that I take Ninja Tabi for the armor they give. If we want different boots, take Beserker's Greaves or work towards Mercury's Treads. Boots are a personal preference. You can even grab Sorcerer's Shoes for the magic pen. Pestilence and Famine both do magic damage, so the pen helps them. Also, that's the only magic pen item you can buy without also spending gold on items that provide AP. Once you have your shoe wear all sorted out, rush a Phage.
Your in a decent place now, and have a few different paths you can take.
1. Finish Trinity Force.
2. Grab a B.F. Sword and work that into a Black Cleaver or Bloodthirster.
3. Build up your MR and health a little bit by building Spirit Visage. If you take this path, grab either Kindlegem first for the health and cdr, or grab the Null-Magic Mantle if your suffering from a lot of magic damage. Build them into a Spirit Visage to increase the health gain from Famine.
If you've gotten this far, start finishing your Trinity Force if you haven't already done so. I like to grab Sheen for the damage proc. If you want the Zeal benefits, grab it instead. Finish up your Trinity and then move on to a B. F. Sword if you didn't do this earlier. This boosts the damage your pumping out on all your damaging abilities- that means more healing, more aoe damage, MOAR damage in general.
You can now turn your B.F. Sword into what ever you like. I tend to grab a Black Cleaver for the armor reduction on hit and the attack speed. Feel free to grab a Bloodthirster if you so choose.
--Personally, I will take the Black Cleaver first. It provides armor reduction- which you have none of, and also grants more attack speed which is nice.
You final item will replace your Amazingly OP Doran's Blade. You have a few options here- you can make what ever you didn't buy before (either the Bloodthirster or in the Black Cleaver). You can also build a Guardian Angel to really screw with people. Watching someone die and come back to kill you multiple times would be rather frightening.

If your suffering from a ton of CC, you can grab Mercury's Treads, Cloak and Dagger, or Eleisa's Miracle early on. Mercury's Treads are a better choice than the others.

I like to take Omen of Famine first. It gives you a little bit of health regen upon casting and something to harass with. After that, Omen of Pestilence for the slow, and then Omen of War as your third ability. After that, lvl Famine to 3, and the alternate between the other two, starting with Pestilence to improve the slow and aoe damage.
*Keep in mind that Famine is the most mana intensive of your 3 starting abilities, so spamming it without Tear can lead to oom rather quickly. If you want to take Pestilence for the aoe instead, feel free to do so. If you happen to be one of them 'In your face' type of undead, take your Q so you can smack people.*
Grab your ult when you can, Omen of Death, of course. After you have them all at 3, keep them even, leveling which ever one you prefer first. Yorick's Passive- Unholy Covenant allows you to deal an additional 5% damage for each summon you have active. If you have all 4 summoned, you will be doing 20% more damage. Try to keep those ghouls out and about.

For summoner spells, I like to take these:
Exhaust & Ignite
I personally feel that Ignite is alright on him and will probably switch it out for either Ghost or perhaps flash. For now, I'll keep these.

Other options:
Flash- Use it to run or initiate.
Ghost- Same as Flash, shorter cd than Flash and you can put a point into Haste if need be.
Clarity- To keep up your mana early, not as useful later on.
Cleanse- If CC has you down, pop this like an allergy pill.
Fortify- If you want to provide more protection for your lovely land lasers. Also gives you additional damage against minions.

1: Dropping Famine on something will heal you for the damage it does initially and then all the damage the ghoul does until it dies. Imagine something like Vlad's Q but less potent.

2: Pestilence is on a longer CD than your other abilities, use it only when you are certain it will slow the enemy. You can always cast it without hitting them and your ghoul will chase after them. You don't have to hit them with the spell to get the slowing effect- as long as the Pestilence ghoul is within range.

3: Your ghouls seem to have their own schedule and will do whatever the hell they please. Mundo Ghoul goes where it pleases. Enemy Priority for Ghouls seems to be:
CHAMPS > TURRETS > MINIONS
I've seen them sometimes continue to attack a tower, while other times they begin beating on incoming minions. In any case, Champs still have a higher priority.
Keep this in mind when you summon a ghoul at an enemy tower thinking it will help you deal damage to it. The ghoul will start attacking a champ if they come close and the turret will aggro to YOU. Try to back off if you have ghouls up and an enemy champ comes up, lest you want to be the turret's new lover.

4: The Omen of War ghoul will not proc on buildings. You must smack a champ or a minion in order for it to summon.

5: You can only have one summon of a certain type out at a time. Casting Omen of Famine while the previous Famine ghoul is still alive will cause it to insta die. You will get a ghoul with replenished health, but you can stack em'.

6. This might be a bug, I'm not sure- I still have to try it in game. Apparently, if you ult yourself or anyone for that matter, and then Mordekaiser throws Children of the Grave on your Cloned champ, he will get a Ghost of the champ you ulted, and the second portion of your ult will not kick off (your ults target will not revive to take vengeance). That's not cool.
--Edit-- I've tried this in game and it's exactly how I've described it above. Don't tell Morde. :\

7. Rylai's Crystal Scepter makes Pestilence and Famine slow upon doing initial damage. DO NOT BUY RYLAI'S. Seriously, just don't.

8. Using Q resets your auto attacks (Much like Nasus' Siphoning Strike). Attack, press Q
immediately after and you will attack again.

9. From what I've read, your ghouls will teleport back to you if they get too far. I've never had one port back to me before, as they never last long enough to get that far. I suppose that if you were to teleport or Blue pill, they might arrive with you and more than likely already near death.

Your laning phase is probably going to suck. I'm not going to sugar coat that fact. If you play dumb, someone will get fed...not going to mention who. Anyway, want you want to do and should always be doing, is last hitting. You need the gold ASAP for your Tear. If an enemy gets a little too close, hit em' with some Famine. Starve them dudes. Try not to take too much damage from being reckless. If you drop below full, throw the Famine Ghoul on some minions or a champ.
Typically, champs will run from it or attack it. In either case, you will not receive as much health as you would have by throwing it on some poor minion. Whenever you get Pestilence, try to use it whenever an enemy comes up to last hit a minion, or just gets close in general. Try to hit them and as many minions around them as possible.
Lastly, your War ghoul is going to be mostly used when you want to push the minions back a little, typically when they are at your tower or are getting close. War is the least mana consuming of your abilities and is on a shorter cd.
If you choose to hit people with your shovel instead, take to the brush, run out and smack them, then retreat back into the brush with the movement buff from the War ghoul.
Last thing- If you are on blue side, head top. Purple, go bottom. The reason for this is that it makes it easier for you to grab blue buff. If you have a jungler, feel free to go to either lane.

You can 1v2, but it probably not turn out well. If you have your ult, cast it on your self either at the beginning for extra damage, or somewhere near the end to get back up at full hp. You will die very quickly after the ult goes off, so make short work of whoever you planned on sticking around for. It is these times that Exhaust is very nice. During mid game, try to jungle or farm as much as possible without becoming oblivious to your teams needs. Try to push towers with others and grab blue buff whenever you can (give it to anyone who needs it more than you). Pretty much do what's necessary to push/stop pushes/ prevent ganks/intitiate ganks. If you have a chance to push an undefended tower, ult yourself, throw a Pestilence Ghoul near the tower, and have at it. Throw a Famine Ghoul on incoming minion waves to keep them occupied and away from you for a few seconds longer. Your ult is on a lower cd than some others and will be up again before you know it.

Stay with your team. You are an off tank, so don't go in thinking "IMA GRAVE DIGGER HARHARHAR" and then die. You will cry, your team with cry, and the enemy will laugh. Let the tank initiate, use your ult either early on or later before the carry dies (about one-third of their health depending on how fast its dropping). If you have more damage than your carry, you might be a better target. Coordinate with your team and keep Baron/Dragon warded at all times. Harass with E, but not so much with W as it puts you very close to the enemy. Press Q then try to get your W off first in fights, press E or smack something and then press E, Q again, W if it's off CD otherwise do E again. Pretty much spam as much as possible- Rinse & repeat.

There you go. If you do well and are raping the enemy team, you've pretty much engaged in some sort of strange necrophilia act. If not, then the enemy has and it's gross either way. If you do well, let me know. Leave suggestions and everything else below. Also, this is my first guide- let me know about any possible changes I can make to the guide to make it better.