This is the build I use when playing Garen, I find it very effective for tanking. With this I can generally make myself all but unkillable but still do enough damage in a fight to be a threat to the enemy team. Hope you like it.

Pros:
Considerable survivability
Decent damage for a tank

Cons:
Garen has no taunt or other disable other then silence

The items in this build provide two advantages for Garen: very high survivability, and a significant amount of damage with only one offensive item.

Now you may not think that Atma's impaler is going to provide enough damage to make a difference but lets do some of the math. With two Warmog's you get 1840hp (920 each) plus up to an additional 900hp (450 each) that stacks as you kill enemies and minions. That comes to 2740hp. Two percent of that is 54.8. So just from the additional HP from Warmog's you get over 50 AD, and that's not counting the base HP of the character or the HP from runes. At 5k hp you are getting 100 AD just from Atma's. Now for a tank like Garen that is enough to cause some damage. In addition to this, his ultimate is devastating to all enemies, dealing base damage plus additional damage based on the amount of HP the target is missing.

The other defensive items in this build should be chosen based on the enemy team, with the exception of the Warmog's Armor. Ageis of the Legion is a great item, and one I always prefer to get unless I am facing an entirely AP or AD team. I usually get this after getting Atma's Impaler and before starting my second Warmog's Armor, and then get my second defensive item after finishing the second Warmog's. The items listed in the build are the ones I recommend for either armor or magic resist.

One comment I get is the lack of a Force of Nature for a magic resist item. The reason for this is that Force of Nature offers nothing to Garen except magic resist. The considerable amount of HP regen that the item has is insignificant with two Warmog's and Garen's passive.
If you are facing a very AP heavy team with lots of nukes you might consider Force of Nature just for the magic resist but I generally find other items more useful.

In combat I generally use the same skill sequence for most every fight. I open with my "Q", Decisive Strike, followed by Judgment and Courage to mitigate damage. Then depending on the HP level of enemies I use Demacian Justice. As a general rule I never use Demacian Justice on an enemy above about 25% of their max HP. This ensures a kill most every time.

With Garen in a team fight you want to make sure to target the enemy nuke or carry right at the start. Your survivability plus the speed boost from Decisive Strike should allow you to blow by the enemy team to get to your target. You want to make sure that as you initiate a team fight you are doing so by silencing the highest damage dealer on the enemy team. This should allow you to negate them in a team fight giving your team time to focus them down before they ever recover from the silence.

Also whenever you use Judgment make sure you are hitting as many enemies as possible. That being said always pay attention to who you are hitting. Don't use it against two tanky character and leave a squishy out of range just to hit more enemies. You could be more effective against the squishy then the other two. Basically be sure to pay attention to your damage output and position yourself in the most effective way when using Judgment in a team fight.

One of Garen's biggest downfalls as a tank is that he has no taunt or other ability to force an enemy to stop attacking an ally. While this is technically true I have found that Garen can in fact get enemies to stop attacking allies.

The key to this is the damage output gained from having two Warmog's Armor and an Atma's Impaler. With this item build you can target an enemy champion attacking an ally and initiate with Decisive Strike to silence them and then use Judgment and Courage. The silience keeps them from using any abilities that might allow them to simply kill your ally flat out and then run away, and your damage output from Judgment is enough to usually make enemies give up on the kill rather then eat all the damage with no way to respond due to the silence.

Now the general reaction that I get to this theory is that most enemies will not simply give up on a kill that easily but I can vouch for the effectiveness of the tactic. Granted there are some players who would rather eat the damage and try for the kill but this also usually ends in failure for them as Garen's damage output is usually enough to get them down to about 25% HP at which time Demacian Justice finishes them off and you walk away with a kill.

Overall I have found this build to be extremely effective with Garen. The amount of HP and HP regen on this build is considerable, and when coupled with Garen's passive, it means that you almost never have to return to base to heal. Just break combat for a few seconds. His damage is considerable for a tank, and the shear killing power of Demacian Justice give Garen a huge advantage in both team fights and one on one fights. I hope you find this build useful. Pleas leave your comments below.