Okay, I'm a new user, I've been using Garen for a quite while now. This build works fine for me when it comes to use it with teamplay; I barely lose (I mean, BARELY. You'll still lose sometimes). Remember; This is my first guide, don't go too harsh on me ^^'

So, this guide focuses on making Garen, the Might of Demacia, an effective off-tank. Contrary to popular belief, a Garen that goes tank build can be very dangerous, given the fact that his resistance, passive, life and ulti can literally kick your *** 1 vs 1 if you don't go well prepared.

There's some points YOU SHOULD READ about this build:
-This build is planned to be the one that group battles must take care of. A Garen can easily kill someone even in a group fight, and do a great difference of power, since his resistances are just too damn high. (As you may guess, This goes tank-like, now keep reading...)
-It may look tank-focused, but originally, this was to keep YOURSELF alive. I wouldn't assure you'd be able to protect your mates in a group fight with this Garen Build.
-Look at that Atma's. Now look the life. Now look back to Atma's. Now the damage is diamonds for being a tank (Which is supposed to endure, not to do damage).
-I didn't mind cooldowns much there. Garen's cooldowns are fast enough, and trust me, I've seen Garens with much lower cooldown fail epically.
-This guide is just a base and my first guide. You don't have to follow it as it is, you can modify some parts of the build, runes, or masteries.
-There may be some mistakes in skill distributions, just have in mind: Level Demacian Justice whenever you can, max Judgement first, then Decisive Strike, and then Courage. Basically, is this.

Pros:
-Say hi to high standing in lane! We can camp there the whole game thanks to our HP and passive!
-Not such a bad damage, for having such a high HP (up to 4,5k!)
-May it be an important factor in Team Fights.

Cons:
-You'll have to abandon your lane many many times. Your mates will need someone to tank.
-Sometimes, you won't be able to protect your mates.
-Given your HP and danger in Team Fights, the enemy team will likely gank you, or attempt to, countless times.

The runes are shown upside, but I'll proceed to explain why these and not others:

-Greater Mark of Desolation: Every Garen should have this. Really. Any questions?
-Greater Seal of Resistance: High defense early game, you shouldn't have any problems.
-Greater Glyph of Shielding: Late game, this will come handy for those AP casters that drive your head off when there aren't any bushes near!
-Greater Quintessence of Desolation: Oh well, no other Quintessence is more handy for Garen than this one.

You can change the defense and magic resistance if you want, but, PLEASE; Do not quit the armor penetration! It's too damn cool for your Q and E!

Also, I may say that some Magic Penetration in Seals or Glyphs would be useful, since Demacian Justice is a magical ultimate, and not a physical. (Thanks Forgottenduty for clearing it up)

This will be short. To farm, you don't have to jungle (In fact, you SHOULDN'T jungle, unless you may need the buffs late game). You just need your holy Judgement to slay those minions off. Sunfire Cape makes this more profitable.

Question time: Some of the masteries shown up don't fit what you guys want, right? There's a reason for that.

For the offense, it would be cool if we leveled Sorcery. There's a problem though, we wouldn't be able to level the armor penetration, so I choose to be a bit faster attacking, which never is bad for any champion!

For the Defense, Vigor may be useful, but having Garen's passive, we don't need more health regeneration, for that later in game we will be a freaking self-hospital.

Indomitable or Evasion. Choose whichever you like of both, but use only 2 points for this.

Why do I level Siege Commander and Enlightement for the cooldowns? Well, most of the time, you will care about guarding the towers (for if the towers fall, you'll lose later or sooner, unless a miracle happens), AND pushing a lane, alone or grouped. This will innevitably get you into the tower-breaking part, and Siege Commander makes the tower more squishy to your Decisive Strike (Which we will focus on later). Enlightement gives that cooldown reduction that everyone likes, and since Initiator is not that good, I choose to enlighten myself :D

-The passive,Perseverance, will heal us the 0,5% of our life each second if we don't take any hit for 7 seconds after our last taken hit. This means, Garen doesn't have to retreat to base, but to hide in a bush (your main point, remember) and wait to recover. This doesn't mean you'll have all your life in 30 seconds, though. If you have to retreat, do so.

-Decisive Strike: Silences an enemy for a given ammount of time and does a decent ammount of damage. Also increases your speed of movement for around 4 or 5 seconds. Perfect to begin a battle!

-Courage: Whenever you are going to get highly damage, use this. Will cover an ammount of damage for 3 seconds (30% at lvl 5! awesome!). Who mentioned anything about not tanking with this?

-Judgement: Or Spin To Win. This thing is a little fearsome for groups early game, and a must-fear for anyone with less than 50% of the life or that just got silenced by Decisive Strike.Trolldameres and Kiddie Yis will **** themselves off if they see you at top (3 vs 3) or mid (5 vs 5, but likely unprobable).

-Demacian Justice: It will take you some time to master this skill, but once you've fully mastered it, barely any enemy will escape from you. Even if you're alone, If the enemy has around 10% or 20% of the life and tries to run away, Demacian Justice must finish them off. You may hear the victim flaming you for doing such an epic kill. Deals 175 / 350 / 525 magic damage plus 1 additional damage per 3.5 / 3 / 2.5 health his opponent is missing. You'll understand when you see it. Also, the cooldown is fast enough to use it before 2 minutes have passed again. It's not recomended, but it serves also to get the kill of a gank, if you need it.

To effectively use these skills, do the following (given the fact that you ambush someone from the bush :D):
1.- Decisive Strike: Silences the enemy, forcing him to run away.
2.- Judgement: To give him more candies :D
3.- Courage: By this time, you may be getting hit. If so, use Courage to minimize the losses.
4.- Keep attaking.
5.- When the cooldowns are off, Q and E again.
6.- Repeat until the enemy has enough low life and many wishes to get his *** out of your attack range, to Demacian Justice him!

-Ghost: Here we have a handy skill. It's useful for chasing enemies down and running away. The main reason we have this and not Flash is to keep hitting the enemy, since Flash doesn't increase your speed AND Ghost makes you unable to crash with any other minions while you chase (Those enemy champions that run away because of a minion barrier... :D)
-Extenuate: Do I really have to explain it? Slows enemies and reduces their armor. In a group fight, anyone who gets this with Garen close to them is practically dead.

OTHER USEFUL SKILLS:

-Flash: If you don't feel like Ghosting, use this. Beware of the cooldown, tho. Also, will help you go through the walls, handy to run away in the case OR chase enemies with flash orTrolldameres :D
-Ignite: Surely, an early-game kill attainer if used well. Mainly for those champions with VERY LOW LIFE. If you used your ultimate into a very lucky dude, Ignite should finish them off. (Also, Trolldamere hates Ignite after his ulti passes off, remember, Endless Rage endures 6 seconds... time it :D)

The other Summoner Spells... Let's just say you may be a noob using Garen if you don't use any of these 4 in your 2 summoner spell slots. May we proceed?

When the game begins, pick aDoran's Sword ASAP. If you need to solo the top, however, aDoran's Shield may come more handy, but that's up to you.

When we get 1.000 gold,Boots of Swiftness will be our selection. Garen NEEDS the speed it gives.

When we advance a bit in game,Frozen Mallet andSunfire Cape will come handy. Buy them in whichever order you want, considering the HP first for the recipes.

*****FROM HERE, CHANGES MAY BE MADE, LOOK BELOW THE RECCOMENDED ITEMS FOR MORE OPTIONS*****
Atma's Impaler andWarmog's Armor Shall make you become fearful. Your HP May be... Insane? +4k. And your Damage will be pretty much decent. DPS's with low armor won't want to face you alone. (Unless they have been feeded by the others).

To finish the build,Guardian Angel not only gives you the Magic resist we lack. As a tank, you'll most likely die protecting your companions or failing at the attack. This will give you a second chance to escape OR act as a bait for the enemy so the others can run away and prepare for the defense.

POSSIBLE OPTIONS:
-Thornmail for those enemy teams with Carry DPS, likeTrolldamere or Kiddie Yi. If the enemy has many melee dps, it's the way to go (if you want).
-Force of Nature When the enemy team is based in Magic Damage.
-Randuin's Omen I've never used this item myself with Garen, but I suppose it's never a bad choose, given the cooldown reduction, armor, life and chance to slow down enemy if hit.
-Stark's Fervor I still think this is an useful item, contrary to popular belief. You should try it once and tell me how it went.

For team fights, you'll likely be HIDDEN in a place that allows you both protect your mates and creep the **** out of your enemies, with the purpose of helping get as many kills as possible. Which's the place to do this? THE BUSH.

-Enemies will likely try to focus someone in your party. Evade this with Decisive Strike and Judgement. If the other mate doesn't run away, it will work.
-This Garen Build relies HEAVILY on the team's behavior. If they don't assist you in ganking someone that you can't kill / got feeded, or your mates themselves are noob feeders, don't expect to win wihout a miracle...
-Having known the point upside, if you're playing with friends and you communicate via Skype or Mumble or TeamSpeak3, this build will work diamonds. (yet, don't expect to win always)
-You're the Tank. You're the tower killer. You're the Dragon Tanker. Enough reason to go help your team in everything they need you for.
-If for any reason, an enemy gank goes successful, and your teammate it's low in life, with you close to him, you can try to save him by extenuating, or silencing the enemy which's stalking. You may die, tho, but that's the tank's duty :D
-Late game, enemy will fear you, since you have high HP (trust me, they will likely go 3 vs 1 ALWAYS on Garen). Use this in your advantage to let your mates advance OR be a bait to lure enemy's attention.
-To end with this, anything that you could think of being Garen to defend or assist.

If you've read this whole guide, then I pretty thank you :D

Suggerences and critics may be welcome.

Insults and kiddies may be erased.

Note that i call Tryndamere 'Trolldamere' and Master Yi 'Kiddie Yi' for real facts. Don't mind me.

I shall update the guide whenever I can :D

Update 1: Stark's Fervor get's moved into optional items, and Guardian Angel get's moved to the main build. Also, some info was fixed (Sunlight Cape instead of Sunfire Cape was fixed, and Demacian Justice information is now more accurate).