Very basic explanation here. As garen your main stat you wanna go for is crit. In MOST games you wont need much survivability more then what you have because courage is insanely awesome.
If your laneing against some good harassers don't be afraid to pick it up early. If your solo lane, stand behind the minions and melee hit over them, with the occasional decisive strike at the opponent for harassment. You can also harass by judgementing over the minions and after the opponent. This is my preferred way of harassing, because it chops down and last hits minions aswell, then you can retreat and persevere back your hp. Once you have DJ [your ulti] you CAN tower dive effectively. Hit the minions infront of it, and then without warning decisive strike and charge them, exhaust them if you have it, and once you hit them, blow your courage to soak damage from the turret. Then drop DJ on them to finish it. Dont try to dive anyone above 50% unless your SURE you can get them.
If your dual laneing, stand behind the minions and hit over your own minions. You can harass by decisive strike judgementing, which means you hit decisive strike and then judgement without using the hit from the strike, and soon as judgement is done you will hit with the strike. You can do that to wear them down whal just avoiding damage and persevering back your hp. If you have someone with a stun, judgement over the stun and if not try to organize a kill when they push. Flank around to the brush and come up behind them. Exhaust helps to get these early kills aswell as ignite, so you should have no problem being fed once you get the hang of it.
In team battles, you CAN initiate. With your descisive strike you can hit a squishy with CC capabilities and then have your whole team jump on them whal their silenced, giving you an early advantage in team battles. If you start to lose you can use descicive to get away. But judgement is one of the better team battle abilities, after you silence, if you judgement, you CANT be snared and are doing damage to most of the enemy team. If the enemy team has tanks DONT WASTE DJ ON A SQUISHY unless its their teams carry [Player who does the most damage to your team]. Wait till you get the tank down to about half, judgement and then DJ. If they dont die, their extremely low on hp and your team shoudl easily clean them up along with the other squshies.
Then trade out the frozen mallet for the last whisper. You should just go pick up the lizard buff for the slowing effect if this happens. If you got a nice feed early, i dont recommend it but it is POSSIBLE to get the blood razors. I dont recommend it because your not an attack speed hero. But it will help to get them down quicker if they have maxed resistances.
If your getting focused trade the black cleaver for warmogs armor, or even guardian angel. If your new to garen you can even do a full survivability build. The survivability build works because garen wont die easily, and as such he can widdle down the enemy hp with his abilities then finish them with his ultumate. If you stack hp and then get the atmas impaler. It super boosts your damage and allows you to stack full survivability gear and still get out good damage.
The Survivability build starts out with dorans shield, goes for mercury treads, then builds the warmogs early for maximum hp early game, which will allow you to push turrets and dive effectively. After that you can get a sunfire cape or two [personal opinion] for aoe damage to stack with your judgement.
SUNFIRE BUILD - After the sunfire you will want to get a phantom dancer for movement and everyhing. And after that another item depending on whats going on.
- Frozen Mallet
- THORNMAIL [Note] If you stack this with atmas impaler, and full sunfire or even full survivability, you will be an EXTREMELY deadly tank.
Non Sunfite Survivability Build -
After warmogs you can get the aegis of the legion for armor and resistances. After that you can pick up soul shroud for the hp and the buff to your nearby champions. AFter this it depends on how the games going. If your getting focused solo, [as a tank, its unlikely] then you should get thornmail, if your NOT getting focused then get the atmas impaler first, followed by thornmail.
I am by no means an expert. But with both the sunfire, and non sunfire builds you can grab the innervating locket for further buffs to your allies considering your tanking.
Its very basic, not too in depth on anything. I dont expect a high rating, its just mostly for the item build for people who know how to play him.