Doran's Blade: I choose this item at the first cause we are building an AD garen so it makes so so op at the early game.
Boots of Switfness: This helps alot to start to kill, when you use Decisive Strike you can reach so easly your enemy, also to escape.
Tiamat: This is the most important item in this build, cause you can farm and jungle easily, and in the middle of the battle you aim 1 champion but if damages also to the close champions and also the regeneration is good.
Black Cleaver: This is to improve the attack speed, also to your skills dmg, and the armor reduction of the enemy.
The Bloodthirsters: this will level up your AD really good, extreme life steal and dmg, when you have the complete build with the bloodthirsters +40(each) you have more or less 450dmg and when you run to a group of enemies with decisive strike you make lots of dmg to all of them and then use spin and you will be so feared.

Also with this build garen comes so great against turrets.
Luck you guys ;) i hope you enjoy and like this build.