Efficient early lane trades and kill secure with [R].
Durability and crowd control
Every 4s your next attack on a champion deals bonus damage, heals you, and permanently increases your health.
After taking damage from an enemy champion, the next 3 spells or attacks you receive deal less damage.
After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Increase your Tenacity and Slow Resistance for each summoner spell on cooldown. Immediately after casting a Summoner Spell, gain a larger burst of Tenacity and Slow Resistance for a moderate duration.
+65 health and +10 AP or 6 AD, adaptive
Gain 3% extra MS. Gain extra AP or AD, based on your bonus MS, Adaptive.
Gain increasing amounts of AD or AP, Adaptive over the course of the game.
Unfortunately Garen doesn't have any fantastic keystone choices, Grasp is just the best of the bad.
You should be able to proc Grasp relatively easily in melee matchups with [Q] and the bonus max health synergizes well with your [W].
Bone Plating massively empowers your ability to take aggressive trades early in lane while mitigation most incoming damage.
Conditioning allows you to scale into the mid game as a very threatening tank.
Strong AD Matchups: If playing into a strong AD matchup like Renekton or Camille, take Iron Skin.
Strong AP Matchups: If playing into a strong AP matchup like Rumble, take Mirror Shell.
The bonus tenacity and slow resistance will help you overcome Garen's biggest pitfall of getting kited by CC and slows.
With Unflinching you should be able to stick to your target for the kill much more easily.
Celerity's ability to massively buff your AD when paired with the bonus movement speed from [Q] and Phage make you a monster carry able to dish out a ton of damage throughout the game.
Gathering Storm help to scare you offensively into late games while still being able to itemize defensively.