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
Brutilizer
Mercury Treads
Trinity Force
Blood Thirster
Sunfire Aegis
Force of Nature
Gaurdian Angel