1/16/2012 - Guide is not 100% complete yet , but the basics have been covered and is what I say " Viable "
1/21/2012 - Updated guide with Video demonstration

Here in this video I demonstrate my build and sequencing , and using Garen , Spells , and Items , to the full extend of my knowledge. Play Video

Hello and welcome to my first guide , featuring Garen , the might of Demacia!
I've been playing Garen since level 10 . He is a great champion , fun to play , bulky and lethal enough to both hold against 2 or more opponents on his own and with enough skill and luck , take them down and jump in for more.
Here I will be explaining some further details on his skills , summoner spells , abilities , Items and some Tips to minimize your deaths , maximize your kills , and make your opponents fear you !

Pros:
-Great solo champ
-Can hold his own against a barrage of attacks
-Has Decent damage
-Is fun to play

Cons
-Useless against long range champions
-Has no CC
-Can be overwhelmed end game

Starting with runes , I use :
9 Greater Mark of Desolation + 3 Greater Quintessence of Desolation
Good armor penetration for early damage

9 Greater Seal of Armor + 9 Greater Glyph of Magic Resist
Bolsters your survivability early game

Perseverance
One of the best passives in the game IMHO , it ensures that you keep laning without having to recall all the time . saves you money , experience and most of all ,gives you an advantage against your opponents. Just stand behind your minions so you wont lose experience , and watch that life bar fill up!

Decisive Strike
Great skill for chasing stragglers , escaping dangerous situations , and canceling enemy ulti's that would otherwise be devastating for you and your team. However I chose to max this skill last since It's not much use in combat its self.

Courage
A great passive , and a fantastic active. it's passive gives you a permanent increase of 0.5 armor AND magic resist everytime you kill an enemy ( Capping at 25 bonus Armor + Magic Resist ) , and when activated gives you a shield that blocks a good amount of all damage taken for a couple of seconds . when properly used this is a life saver , for example , using this against Gangplank's Parrrley will shave alot of edge from it's otherwise high damage , and leave GP open for assault. I chose to max this second .

Judgment
This is your greatest skill .It helps you farm quickly , it does continuous damage to surrounding enemies and REMOVES ALL SLOWS . Do you know how important that is ? Yes you do ! Couple this with Decisive Strike + Boots of Swiftness , and you are going to be a hard bird to catch ! I chose to max this skill first , as it is both your greatest offense skill , and your second best escape skill, not to mention , a damn squishy killer move endgame!

Demacian Justice
Ahem. DO NOT INITIATE WITH THIS SKILL . DO NOT EVER INITIATE WITH THIS SKILL . EVER. It is very powerful , and your opponents fear you for it ! However since it deals extra damage , depending on how much health your target is missing , it's the stupidest thing ever to initiate , counting the fact that it has a looong cooldown , you will be exposed
in 1 versus 1 fights and getting the hell beaten out of you like a sitting duck . Always use this skill when your target is below 40% - 35% health

The first item you're going to buy is Doran's Blade. It gives you attack , health , and a small ( but good for early game ) amount of lifesteal which is a pretty good kick-off for farming efficiently using Judgment. Why I chose not to buy Doran's Shield? . Garen already has enough armor at this point + his passive health regeneration is adequate . At the hands of a skilled player he is just as effective without it.

~Personal Note
Q: Should I stack daggers or not?
A: It depends. I prefer to save my money so I can rush Boots of Swiftness and Frozen Mallet And all the sweet stats and effects that come with it. But if you wanna boost your damage early game then go ahead!

Next , if you're going to be solo top , you will rush Boots of Swiftness. Pair that with Decisive Strike and you will be almost un-gankable. Otherwise , if you have a laning partner , then pick Boots and Ruby Crystal so you can build Phage Afterwards.

One you have Boots of Swiftness and Phage ready , then your next item should be Giant's Belt and if you have enough money turn Phage and Giants Belt into Frozen Mallet . Frozen Mallet is a kick-*** Item , since it gives you both attack and LOTS of health , but also makes your attacks slow your opponents by 40% which is a great deal overall!

Afterwards you should build The Brutalizer.One of the main reasons I pick brutalizer is
so I can ease in a bit of the skill cooldowns. Cooldowns is one of Garen's great weaknesses , and if you fail to manage your skills properly you will be left in the cold , since Garen is not a good DPS champion and is very skill-reliant. Next , you should get a Chain Vest and another Giant's Belt in order to build a Sunfire Aegis , that gives you 45 extra armor ,stacks another 400 health , PLUS 35 dmg per second to surrounding units , which makes mass farming even easier at this point.

Note~ You can always turn your The Brutalizer into Youmuu's Ghostblade whenever you have the chance

Now , you should have enough health to buy Atma's Impaler which gives you an additional 45 armor and also , Atma's unique passive converts 2% of your max health into extra attack damage So , by now you will have around 210 attack damage.

Note~
If you find yourself being overwhelmed by magic damage , then I suggest you buy a Negatron Cloak so it cuts some edge off of it. If you still think that Magic damage destroys you then you can go ahead and get a Force of Nature. Even though it's an expensive Item , it will definately cut down the magic damage you receive for good.

So by now you will have enough armor and health , to be able to take punishing from 3 champions and still survive. But , you might find yourself lacking in damage.
The next step is to buy a B.F Sword , a Vampiric Scepter and turn them into Bloodthirster . The bloodthirster stacks attack damage and lifesteal each time you kill a minion , and you lose all of your stacks upon death. However , since you are tanky enough to be hard to kill that wont be much of a problem if you play your cards well in team fights.

Note~ If you find Bloodthirster to be unreliable , then you can always buy Infinity Edge instead. I personally prefer Bloodthirster though , due to the amount of lifesteal it gives you.

This concludes your build . With the bloodthirster in hand you will be able to take the fight into the enemy nexus and with your team , take the victory.