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.
This is my first build, thought I'd give it a try and see how it works/what other people think of my play style. Mostly when I play as Garen I go for the items listed in the order they are eventually selling Doran's Blade for the Infinity Edge. I buy the Doran's Blade first instead of the shield because early on Garen's passive does a pretty good job of keeping you alive as well as the health potion and the tiny bit of lifesteal the Blade gives you. I choose the Blade as the idea is that it should help you get the First Blood or at least the first kill in your lane. With the amount of armor pen. the runes give you and the relatively high damage your Judgment does at the beginning it should work. After the Blade pretty much roll straight tanking gear and play as a tank. Help your teammates and don't worry about initiating team fights; but don't charge in like and idiot either. Once you have Warmog's Armor and Force of Nature you will have a ton of Health Regen with your passive so start a fight, survive, use your Courage to diminish the largest blows of the enemy, then if you get low exhaust and ultimately flash out, regen, then run back in. Your ultimate is a very powerful move that is not modified as much by your Attack Power as it is your opponents lack of HP. For any enemy champion at or below ~%30 Health it is pretty much a one-hit kill. That is what you want to be looking for when you start team fights. Don't blow it on the first squishy DPS that your team targets, don't be a greedy jerk; instead save it for the end of the team fight when one or more enemy champions might be trying to get away, or when your team is running away and that dps thinks he can get one more kill off your team.... THEN hit them with Demacian Justice. Other than that, Courage can also be effective at tanking towers, just know that you alone cannot tank a tower as your dps with this build sucks; you are a tank and a combat finisher, nothing else.
And for everything else, just have fun.