Garen is an early game powerhouse and with this build, his playstyle becomes dynamic and require more decision making. His role in the team is to wreck his lane (top or bottom lane) so hard that the enemy won't be able to recover. Garen is not a tank and should not be built that way. Garen is essentially an early/mid game assassin and late game anti-carry. With this build you should not be initiating fights unless you know you're target will die and you can escape alive.

Greater Mark of Desolation:
Self explanatory. Standard runes for AD type champions. I will not go into the details of armor penetration benefits and calculations because there are other guides that explain it much more effectively.

Greater Seal of Resilience:
Flat armor runes help you greatly in your early game dominiation. It helps you tank the creep damage for the first several levels which is important because you want to be playing aggressive to deny your lane from creep kills and experience.

Greater Glyph of Resilience:
Garen is one of the few champions that can full out turret dive at lvl 2 and get away alive and with a kill. I use even more flat armor to increase my chance of surviving a tower dive. If you are playing garen and you are not playing aggressively enough to tower dive for a kill at early levels you are not fully using Garen to his strengths.

Greater Quintessence of Strength:
These quints are geared towards early game domination and it helps you with last hitting while denying your lane opponent at the same time. You can easily kill a squishy champion like ashe at level 2 with your QE combo.

I prefer a 15-0-15 setup for a balance between damage and utility.

Archaic knowledge is incredible on Garen. Why? Because his ultimate ability is entirely magic damage. a 15% magic penetration on his ult could a dead Cho'gath and not a Cho'gath running away with 50 health. I will be attaching a Garen ultimate calculator very soon and you can play around with percentage magic penetration to see the difference for yourself

Improved flash and movement speed allow you to chase down your targets because Garen does not have an instant gap closer.

Maxing Brute Force in the mastery tree along with my AD quints and a Doran's blade, I start out with 80 total AD at level 1. That is incredible burst and will strike fear into your opponent.

Doran's Blade Great item on Garen for early game survivability and damage. Build 2 or 3 after boots of speed to wreck your lane even harder.

The Brutalizer With 2 or 3 Doran's Blades and this item you will be able to kill an entire minion wave with Judgement. Oh also you can kill Ashe with a single Q -> E.

Mercury's Treads Getting stunned or silenced is your greatest enemy. Garen depends on his speed burst gap closer to engage or run. Garen also depends on his skills more than his auto attacks. This item is extremely important for not only surviving, but most importantly retaining your DPS potential.

Trinity Force Why this item when Garen does not use mana? The reason for this item is because of Garen's Decisive Strike (Q). This item makes Garen's gameplay very dynamic. Instead of mindlessly spinning in and hoping the other team will throw all their abilities at you, you want to be dodging in and out of fights avoiding focus and use your Q to nuke down casters and AD carries.

Bloodthirster Again, Garen's Q makes bloodthirser amazing. You will be suprised how much health you are healed for with a single Q with sheen proc.

Sunfire Aegis It is Garen. He needs that cape of suns

Force of Nature Magic resist is just a bonus with this item. The real reason for this item is health regen and movement speed. WIth Trinity Force movement speed bonus and merc treads, once you get this item ain't nobody gonna stop you from running up to them.

Guardian Angel The game should be over before you get this item. But if it is not, this item is game changing. You can play Garen extremely aggressive and be even more OP

Most Garen's get Courage(w) at level 4 but not this Garen. This Garen is a real man. Maximize your damage to wreck them so hard early on that you won't even need to use courage later.

Ignite:
At level 2, Q + E + Ignite + Auto attacks will kill anyone with 600 health or less

Flash:
Save and use your flash offensively. It will get your kills. Real men don't use flash to run away.

Many times I have seen Garen players focus so much on dealing damage that they fall behind on cs. This is something you do not want to do. League of Legends is basically farmville with better graphics. Many opponents I have faced say "Oh Garen you are OP now but wait till late game I will wreck you." NA AH. Not if I deny you cs while having at least 70 cs by 10 minutes. There isn't much "falling off late game" from that.

Farming efficiently with garen means knowing how much damage your Judgement(E) does. You can do the math every 3 minutes when creep stats change or you can just learn by experience. Usually in an average game, by the time you get The Brutalizer you will be able
to kill ranged minions with 1 spin
to kill melee minions with 1 spin + 1 auto attack
to kill canon minions with 1 spin + 1 Q

Most importantly do not neglect farming when playing the bully in your lane. 6/0 by 20 minutes but with only 50 cs means you're not really that far ahead compared to an Ashe who is 0/3 but 120 cs.

These examples show how Garen is played using this build.
(Coming soon)

Farming
Trinity combo
Flash combo
Ignite ult combo
Teamfighting