I've been playing Garen a long time and he is by far my favourite champion however he's not perfect so I'm always trying new ways to make him stronger and this what I've found works well so far armor pen and scaling health regen runes.

This build focuses on 1 of Garens main strengths his health regen and his overall durability the main tools/items of this build the armour pen, speed/slows, health regen and items that allow push lanes without having to be on the lanes your self.
I primarily play this class as a damage dealing off tank, strong enough to lead the fight, but quick enough to chase down kills, this build is about balance and making most of your tools in your inventory.
The main thing to remember about this build is that you will heal faster than most others and usually when fighting if you can still have most of your health while they have little to no mana, you will win.

Make the most of your health regen and please try this build and please leave comments below. Lastly beware of Fiora and Tyndamere if fed they will give you the most trouble try to ban them whenever possible.

This build plays to Garen's strengths 1 of which is having health regen runes stack on top of his health regen passive. However regen alone wont will you lanes the shield/barrier spell will allow to stay alive when you expect to take life ending damage and take less damage when engaging the enemy champions during skirmishes.

Ignite should help you finish them of when they decide to retreat after you use your shield/barrier to avoid critical damage.

This will be very useful during early laning. The most useful weapon in your inventory will be the combo of Youmuu's No Ghostblade and Frozen Mallet the slow and speed boost combined should allow you to chase down and secure and kills during the mid to late laning phase.