Garen's health regeneration greatly increases due to his passive, Perseverance. This only activates if he can avoid receiving damage for 7 seconds. This ability gives him exceptional durability in lanes.
Garen can be lethal in early game fights, especially after getting his ult. 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.
Last hitting minions with Courage can help permanently increase your armor and magic resistances.
Decisive Strike can be used effectively to push turrets, as this ability also affects turrets.
Probably the best starting skill is Garen's third ability, Judgment, removes slowing effects on him and reduces the duration of new slowing effects by 50%. Activating this ability just after a Decisive Strike can give you the speed boost and the slow reduction you need to get away.
Because of good early game damage with Judgment, Garen lanes very well with a partner with early game crowd control such as Sion with Cryptic Gaze or Taric with Dazzle.
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.
Beware that Demacian Justice can be blocked by Banshee's Veil.
Demacian Justice can be activated during Judgment on an enemy at low hp. This will cause Garen to imediately cast it after finishing his spin. As long the the enemy Champion remained near you when you activated Demacian Justice, you can kill that champion, reguardless of how far away they are.
Build UsageEdit
Garen has no ability costs making cooldown reduction very strong, The Brutalizer is a good early item for him.
Since Garen's damage is purely physical (except for his ultimate), armor penetration items greatly benefit him.
His Judgment does benifit from Critical Strike, making items such as Phantom Dancer increase the damage output of that ability as well as his basic attack.
Garen does not get any bonuses from ability power, rendering it completely useless on him. He can, however, choose between damage/critical strike and tank roles, having excellent affinities for both of them.
He does, however, benefit from magic penetration such as Archaic Knowledge, due to Demacian Justice dealing a high amount of base magic damage and Sunfire Cape being a core item to any build.
Tank items can make Garen a very effective tank in combination with Courage. Also, Sunfire Cape can work really well with Judgment and items like Leviathan are very useful to him as he can get a lot of assists.

Decisive Strike refreshes attack cooldown on cast.
Decisive Strike can't deal critical damage or critical strikes.
Judgment is not channeled, meaning it can not be stopped by silence, stun, or taunt. These will only stop Garen from ending Judgment to use the attack from Decisive Strike or Demacian Justice.
Judgment can deal critical damage, but only the bonus damage gets multiplied in that case, leaving the base damage unaffected. When Judgment deals critical damage, a sword particle will appear above the enemy.