Throughout Valoran, the resolve of Demacia's military is alternately celebrated or despised, but always respected. Their "zero tolerance" moral code is strictly upheld by civilians and soldiers alike. In combat, this means Demacian troops may not make excuses, flee, or surrender by example. Garen, the valiant warrior who bears the title "the Might of Demacia", is the paradigm to which these leaders are compared. Thousands of great heroes have risen and fallen on the bloody battlefields between Demacia and its preeminent rival, Noxus. It was beneath their mighty banners of war that Garen first met steel with Katarina, the Sinister Blade. The infantrymen who beheld this event (and survived) commented that it seemed as though the two were locked in a mortal waltz set against a symphony of clashing blades.

Garen, the pride of the Demacian military and the leader of the Dauntless Vanguard, returned from this battle breathless for the first time in his career, though some speculate this was due to reasons other than exhaustion. The plausibility of these rumors was bolstered when, in every instance thereafter, Garen seized the opportunity to encounter the Sinister Blade again. A paragon of Demacian ethic, Garen never entertained such allegations, for he knew others couldn't understand. Even simply the pursuit of a worthy opponent on the battlefield is, to a true warrior, the reason to rise each morning. The promise of one, particularly one so beautifully and diametrically opposed, is the validation of his existence.

"The most effective way to kill an opponent is to slice through the man next to him."
― Garen, on front line strategy.

9 Greater Mark of Armor Penetration
9 Greater Seal of Armor
9 Greater Glyph of Scaling Magic Resist
3 Greater Quintessence of Attack Damage

21-9-0 For lane bully
9-21-0 For defensive play

Perseverance Garen will regenerate 0.4% of his maximum health every second if he has not been hit with an enemy ability or taken damage from any source (excluding minions) in the last 9 seconds.

Decisive Strike ACTIVE: Garen breaks free from all slows affecting him and gains 35% movement speed for a few seconds. Additionally, his next basic attack within 4.5 seconds will deal additional physical damage and will silence his target.
BONUS PHYSICAL DAMAGE: 30 / 55 / 80 / 105 / 130 (+ 40% AD)
MOVEMENT SPEED DURATION: 1.5 / 2.25 / 3 / 3.75 / 4.5
SILENCE DURATION: 1.5 / 1.75 / 2 / 2.25 / 2.5

Courage PASSIVE: Garen's bonus armor and magic resistance are increased by 20%.
ACTIVE: Garen gains a defensive shield for a few seconds, reducing incoming damage by 30% and granting 30% crowd control reduction for the duration.
DURATION: 2 / 3 / 4 / 5 / 6

Judgment ACTIVE: Garen rapidly spins his sword around his body for 3 seconds, dealing physical damage each second to enemies within range. Minions take 75% damage from Judgment. Garen ignores unit collision while spinning, but moves 20% slower when traveling through minions. After 0.5 seconds, Garen can reactivate Judgment to cancel it early.
PHYSICAL DAMAGE PER SECOND: 20 / 45 / 70 / 95 / 120 (+ 70 / 80 / 90 / 100 / 110% AD)
TOTAL PHYSICAL DAMAGE: 60 / 135 / 210 / 285 / 360 (+ 210 / 240 / 270 / 300 / 330% AD)
Judgment can critically strike, multiplying the total AD component by (Garen's critical strike damage modifier − 50%).
DAMAGE PER SECOND WITH BASE CRIT MODIFIER: 20 / 45 / 70 / 95 / 120 (+ 105 / 120 / 135 / 150 / 165% AD)

Demacian Justice ACTIVE: Garen calls upon the might of Demacia to attempt to execute an enemy champion, dealing magic damage plus additional damage based on his target's missing health.
MAGIC DAMAGE: 175 / 350 / 525 + 28.57 / 33.33 / 40% of target's missing health

Garen is a melee champion, who excels in quickly killing priority targets and being able to soak up tons of damage.
Damage
The Brutalizer is a good early item for him and can later be built into The Black Cleaver, which is very effective on Garen as Judgment can quickly apply stacks on multiple enemies.
Another option would be Youmuu's Ghostblade; it gives similar AD and Armor penetration, but it also gives critical chance and a short boost to movement and attack speed. This makes it a great late game item when critical chance outscales raw AD and Garen can use the extra attack speed to help destroy turrets.
They can also be built together for an easy source of 20% CDR to combine with Spirit Visage. This also increases wave minion clear due to minions having low armor values.
Last Whisper is a good counter to teams stacking mass amounts of armor but gives no defensive stats, which are important if you are to survive the enemy's damage long enough to deal yours.
As both Decisive Strike and Judgment benefit from critical strike, items such as Infinity Edge increases the ability's damage output as well as his basic attacks.
Atma's Impaler can be a decent item to finish Garen's build, as it gives AD and crit chance along with armor.
Maw of Malmortius is a good anti-mage item. An early Hexdrinker can help Garen against high burst magic damage.
Resistances
Courage scales with Garen's armor and magic resistance; therefore, purchasing items with said stats makes the ability more effective.
Spirit Visage synergizes well with Perseverance, easily allowing him to regenerate 0.48% of his health each second.
Frozen Mallet is an excellent health item for Garen, as it provides some attack damage and a powerful slow to help Garen stick to his target. It also synergizes well with Atma's Impaler.
Warmog's Armor can be a good choice as well; the item's passive along with Perseverance allows him to regenerate 0.6% of his health per second, allowing him to recover health quickly after teamfights without going back to base. The extra health along with Courage makes him very hard to kill in a teamfight.
In combination with Judgment, an early Sunfire Cape helps with Garen's wave clearing capabilities, as well as providing mixed damage and constant threat.
Randuin's Omen can help greatly against being kited and reduces the enemy ranged carry's DPS. An early Warden's Mail is good against auto-attack reliant champions such as Kayle, Xin Zhao, Fiora, etc.

Garen's strength as a bruiser comes from his passive, Perseverance and Courage to keep himself healthy along with his decent early damage and utility from Decisive Strike and Judgment, with Demacian Justice providing a reliable and powerful execute.
Garen's health regeneration is greatly increased due to his passive, Perseverance. This only activates if he can avoid receiving champion or monster damage for 9 seconds, granting him exceptional sustainability in lane as minions do not stop this effect.
Garen's Decisive Strike makes him a powerful chaser, easily closing the gap and silencing enemies before they can cast a slow or any debuff. It can also be used to make a swift escape since it removes slows.
This makes Garen very strong against most caster-type champions due to its silence; disrupting combos can save teammates by preventing casters (both AP and AD) from comfortably using all their skills before they endanger themselves, especially since Garen himself is naturally tanky enough to rush in with little casualties, be it 1 v 1 or in a large teamfight.
Decisive Strike can be used effectively to push lanes, as this ability also affects structures (turrets and inhibitors).
Using Decisive Strike to catch up with, silence, and deal damage to a fleeing champion is the perfect precursor to a killing blow with Demacian Justice.
Decisive Strike can be used to return to/move between lanes quickly, as it has no mana costs and a fairly low cooldown, especially at higher ranks.
Decisive Strike resets Garen's auto-attack timer, effectively allowing him to deal good damage in a short period of time. However, Garen will have to come to melee range without a speed boost if he uses the skill to double-attack.
Garen can be lethal in early game fights, especially after getting his ultimate. Judgment deals good continuous damage to enemies and Decisive Strike's speed boost is good for chasing enemies. Demacian Justice also does extra damage based on how much health the enemy has missing
Garen's best starting skill is Decisive Strike, as it can be used to last-hit, silence, speed himself up, remove slows, reset autoattacks and has very high burst damage at early levels.
Courage and Decisive Strike mix well for a powerful chasing and escape combo. The speed boost from Decisive Strike allows you to get in or out of range, while the damage reduction of Courage allows you to mitigate any retaliation from enemies.
Because of his strong early game damage from Judgment, Garen lanes well with a partner with an early game stun such as Sion's Cryptic Gaze or Taric's Dazzle, though he is most often played as a solo top laner.
Unlike other abilities, Judgment remains active when Garen is stunned because it is not a channeled spell.
With some points into Judgment, Garen can easily farm and push minions with it.
Judgment is Garen's main source of damage, but has a significant cooldown and long duration to maximize its damage. Make sure you activate Judgment properly or you will be left being fairly ineffectual in combat while it is on cooldown.
Beware that Demacian Justice can be blocked by Banshee's Veil and its damage depends on the enemy champion's magic resistance and missing health.
Demacian Justice can be activated during Judgment on an enemy at low health. This will cause Garen to immediately end his spin and cast if the target is in range.
While any champion benefits from brush control, Garen excels at this due to the threat of his gap closing and damage capabilities; use this to your advantage, especially against ranged opponents.