Garen 3 Basic Knowledge:
1. 7sec out of combat, regenerate .5% of his max hp.
2. Justice give bonus damage base on critical chance.
3. Demacian Justice a is magical damage that gives extra damage depend on enemy champ missing hp.

Runes:
Focus on armor penetration for Marks and Quintessence due to all his attack and ability are physical except his ult, then get critical chance for all other runes to help damage and Justice critical proc.

Summoner Spell:
Ignite- works perfectly w/ Justice or make sure they die or stop Mundo's Ult or remove Mordekaisers armor.
Exhuast- w/ 1 point on cripple you have 2 type penetration bonus and slowing effect, meaning they well surely be in great damage. Also allow you to pin them down w/ out you chasing them too much for 3.5 sec, which in all your Justice and Ult is a guarantee killer move. But remember if they got flash, make them use theirs before using exhaust, it well b a pain in a butt and waste of spell.

Mastery:
<Offense>
3 Deadliness- gives Justice bonus dmg.
1 Cripple- gives 2 type penetration plus .5 sec duration
4 Sorcery- helps a bit from cd
3 Sunder- all attack except his ult is physical
3 Brute Force- a little dmg boost for early game
3 Lethality- very helpful boost damage w/ his physical.
1 Havoc- w/ Lethality its 15% boost, its alot of damage.
<Defense>
3 on armor and 3 on resist helps in early game.

Item:
Start w/ Boots of Speed and 2 Health Potion to help you move around faster and last long in lane till your able to buy Giant's Belt. Get your Sunfire Cape first before you get Boots of Swiftness which in all perfect for farming. Buy Averice Blade to help you accumulate gold to get B.F. Sword, then Pickaxe, then Cloak of Agility and finally combine them into Infinity Edge your main source of damage. Purchase The Brutalizer but don't combine Avirce Blade yet cause you still need accumulation gold to get your Atma's Impaler. Once you complete Youmuu'sblade get Zeal and finalize it with Phantom Dancer. (Note: Buy them piece by piece as much as possible try avoiding buying them in full unless you mad a lot of gold when you back.

Comparing Items and Reason:
1. Boots of Swiftness > Mercury's Treads : BoS allows you to move in fights and escaping with Decisive Strike while MT only save you from cc and resist but they can still catch to you, remember your DPS not tanking.
2. Sunfire Cape > Warmog's Armor : SC is constant 35 magic damage when your in range which you are always at melee range when you fight. WA is good for Atma's Impaler but do you really need the extra 35 damage or get the constant 35 magic damage from SC.
3. The rest of the item has no pair to compare unless your building hybrid or tank.