Perseverance (Passive) Garen regenerates a portion of his maximum health every 5 seconds. This regeneration process is set on a 7 second cooldown, when hit by an Epic Monster, Tower, Champion.
Decisive Strike (Q) Garen gains 30% Movement Speed. His next basic attack within 4.5 seconds deals bonus Physical Damage and silences the target for 1.5 seconds. This Ability can crit and proc Energized Attacks from Items like Rapid Firecannon or Stormrazor.
Courage (W) Passive: Garen gains 0.25 bonus Armor and Magic Resist per minion killed. Stacks up to 30 times at 120 minions. Active: Garen gains a shield for 10% of his maximum health and 60% tenacity for 0.75 seconds. He also receives 30% less Damage for the next few seconds.
Judgment (E) Garen starts spinning and dealing AOE damage to all nearby enemies for 3 seconds. The nearest enemy takes 133% the damage. Amount of spins scales with Attack Speed.
Demacian Justice (E) The targeted enemy Champion takes damage equal to 150 / 300 / 450 plus 20 / 25 / 30% of their missing health.

Trinity Force Probably the best Item for Garen as of right now. Even though it contains Mana in its build, which is useless for Garen, it's still incredibly worth it. It comes with Phage, a good Item on Garen because his E Judgment can proc its passive. Then there's Stinger which grants a good amount of Attack Speed and 10% Cooldown Reduction, both of which is very useful for Garen's current Kit. Then there's Sheen - I'm sure a lot of low elo players don't really understand how strong this Item is, as it's incredibly strong for Garen's main combo (more on that later).
Phantom Dancer Phantom Dancer is a very strong Item on Garen eversince the Rework - it comes with everything Garen needs - loads of Attack Speed, Crit and a very valuable Passive called 'Lifeline', which shields you when your HP hits critical levels. It's basically the ADC counterpart of Sterak's Gage.
Boots Boots of Speed are obviously a must-have on every Champion. But Garen has quite a few Options to choose from. If you're opting for the highest possible damage take Berserker's Greaves. Otherwise take Ninja Tabi or Mercury's Treads depending on matchup or enemy composition. Against lane bullies you always take Ninja Tabi or Mercury's Treads.
Guardian Angel One of the best Items out there - optimal on Garen. Grants AD, Armor and a Passive that revives you whenever you die (5min cooldown). Makes you more viable as a frontline in teamfights; run in and AOE hit as many people as possible. With W passive and Guardian Angel's Armor you can peel very well for your other teammates. You're probably going to die from such a kamikaze attack - but no worries, Guardian Angel will revive you.
Dead Man's Plate Makes you a lot tankier, gives you more ability to peel for your team. Also provides a great passive to make you more mobile, combined with your Q Decisive Strike you're going to be a fast boy!
Spirit Visage Seems like a normal Magic Resist Item on the surface. But the real precious part about this Item is its passive: the 30% increase of self-healing effects, which includes: Garen's Passive Perseverance, Ability Healing through Ravenous Hunter and Physical Damage Healing through Death's Dance. It's all coming together, isn't it?
Death's Dance This in combination with your E Judgment turns Garen into a Lifesteal Frontline beast, less effective but similair to Aatrox or Xin Zhao. As hinted before, Ravenous Hunter and Spirit Visage can be used to maximize this effect.
Infinity Edge This is going to increase your Critical Strike Chance to 50%. It also provides incredible amounts of AD. What else do you want? (Only works when far ahead or as ADC Garen)
Thornmail Very useful mid-lategame Item when playing against a lot of AD, especially if there's a fed adc in the enemy lines.
Last Whisper Very useful Item when playing against a lot of tanks, or only one incredibly chunky boy even. Always worth picking up and continueing into Lord Dominik's Regards or Mortal Reminder when facing a lot of healing.