Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
When the game starts hurry up and buy your dorans shield and get to the bushes in your lane with your partner, preferably one with a stun. Usually you can get a kill really early game if u can judgement on them for all 3 seconds and the easiest way to do that is to wait in the bushes.
There is no need to get attack damage or anything like that on garen. He has more then enough damage output already and attack damage would just be wasted on him due to his slow attack speed. Judgement is really his bread and butter skill and it can bring down groups of people.
Decisive strike is great for that silence, making it all that much easier to take down targets like ryze, karthus, kassadin, and annie. Also, if you activate decisive strike before u use judgement, the speed bonus applies while ur spinning so its easier to keep up with the enemy, and you can get off a quick decisive strike after. So it goes:
Activate decisive strike -> Judgement -> Hit decisive strike -> Ult if necesary
With all the extra damage from sunfire capes, just being near an enemy is good enough, be careful around towers though because even if you dont attack an enemy the sunfire damage makes a tower target you.
Force of nature is great just for that extra magic boost and it gives u enough movement speed to keep up with your opponent trying to run away from judgement.