Welcome to my guide! Lets get things started shall we? First off, this guide(or at least the items that I used in this guide) can work for any tank in the game. Except for Madred's Bloodrazor and Banshee's vail.(Since not all tanks use mana) Also, this guide WILL not make you the best Tank Warwick out there(It possibly could but it's not likely) Warwick is a pretty easy champ to get the hang of. He has great survivability(especially with this guide) and he has a personal heal. Goodluck to all of you fellow LoL players, and have fun!

Please try out build before commenting or voting. If you have any suggestion please let me know by either leaving a comment or sending me a message.

Thanks

Pros:
Great survivability
Easy harass with Q
Ult suppresses target(CC)
Easy to figure out
Is pretty flexible(meaning he can be AD, AP, Tank, Hybrid.)

Cons
Very mana hungry early game
Ult can be stopped with CC or cleanse

I prefer to use flash and exhaust for the reasons:

Flash allows you to escape quickly if needed, also good for ganks.
Exhaust can either keep people from getting away, or can diminish the damage coming from an AD champion.

Other good choices are:
Ghost-Goes great with your E, making it nearly impossible for anyone to escape you.
Teleport-Great if you have to go back to base and then have to hurry back to your lane. Also useful for ganking because of the fact that you can teleport to wards.
Ignite-Useful for getting those people that get away with barely any HP. (I don't see much point in this if you're going to have ghost)
Absolutely positively do not get these:
Rally-Want to down the tower in under 10 min? Go ahead I guess....
Revive-This is only useful if you die, if you plan on dying a lot then go ahead...
OK choices:
Heal-Could save your life a lot, could also save an allie.
Clarity-Never go OOM, and never let your allies go OOM.
Clairvoyance-Ok spell, best to save this for your support.
Fortify-Lets your towers stay alive longer.(Also useful for those pesky towerdivers because of the increased attack speed)
Cleanse-Useful if your fighting against a team with a lot of CC.

The runes I chose and why I chose them, well, its quite simple actually.
Greater Seals give the most armor(exception for Quints)
Greater Glyphs give the most MR(once again exception for Quints)
Get 6 of each, and then get 3 Greater Seals of Fortitude and 3 Greater Glyphs of Fortitude and you have a pretty good start. With the marks you get 3 armor,3 MR,3 HP. Pretty simple.
Quints you get one of each.(One Armor per level, One MR per leve, One Flat HP)
Reasoning behind getting Armor per level and MR per level runes: Most champions do not have a whole lot of damage at the beginning. Simple enough. In the end you have more armor and MR with these than you do the flat armor and MR runes.

Most of the items are items that I would get for any game. But, there are a few that could switched out that would work better in different situations. For example, if you have 2 heavy AD enemies and 2 heavy AP enemies, the build I have listed would work best. But, it isn't always like that. Sometimes you end up fighting against teams that have 1 AD champion and 4 AP champions. In this situation you would want to switch out some of that armor for MR. Knowing what items to get every situation is one of the main keys to mastering the position of being a tank. If you're having trouble deciding what to get, I suggest you take a look at SquirrelPoower's guide: Tanking, The under-played role. It will give you a few tips tanking. And for off tanking I suggest checking out DuffTimes guide: Offtanks, The New Metagame.

Well I think that raps it up. I hope this guide helps you out and that you take those assists to the max! Goodluck and have fun!