Garen is a very versatile champion in the middle and end of the game.
With this build and these strategies, you will dominate in PvP, being able to hit criticals up to the QUADRUPLE digits.

Also, with the high damage, life steal, and armor passive of Courage, Garen is naturally powerful in the health and damage areas.

Ignite: Good for getting that kill in.
Exhaust: Good way to catch up to and weaken an enemy champion.

Sword of the Occult: You can change this weapon to a Phage, but if you get fed, Occult is much better.
Buy Pots and Sell Doran's if you need to!
WHY I CHOSE THESE WEAPONS
Doran's Blade: A good starting weapon, and good to sell if you need money.
Boots Of Speed: Goes into Berserker's Greaves.
Long Sword: A decent weapon and goes into the Sword of the Occult (or Phage).
Berserker's Greaves: Attack Speed and Mobility.
Sword Of The Occult: Gives bonuses if you kill or assist.
ALTERNATIVE ^^ Phage: Slows and gives health and AD.
B.F Sword: AD And goes into lots of good weapons.
Avarice Blade: Gives coins and Crit Chance. Also goes into Yoummu's Ghostblade.
Yoummu's Ghostblade: Lots of reasons, one being shortened cooldowns.
Infinity Edge: TONS of AD and Crit Chance.
B.F Sword: Already Mentioned.
The BloodThirster: Lifesteal and AD.

PROS:
-Good KS and good for farming.
-High damage output.
-Crit Damage is 1k.
-Tanky mid and late game
-Silences.
-Able to secure kills with Demacian Justice.
-Manaless
-Able to remove slows with Judgement.
-Very easy learning curve.

CONS:
-Hard to go solo.
-Cannot take down a rush very effectively.
-Dependent on snares and stuns to kill faster opponents. (Unless if you are using exhaust)
-Item dependent.

-Be cautious early game (0-5 min); you are very vulnerable to enemies with snares and stuns such as Ryze and Sona.
-Use these skills in order Courage>Decisive Strike>Judgement (and if necessary) Demacian Justice.
-Use Exhaust and Ignite to your advantage. Exhaust to catch up to faster opponents and Ignite for low health enemies out of your melee reach.
-Use Judgement and Decisive Strike to harass and most likely save an ally being chased by an enemy.
-Decisive Strike can quickly destroy a tower if upgraded.
-Apply Demacian Justice only when enemy is at low health.