Garen, the sunfire tank, the useless dps you ignore because Garen hits like a noodle. The inside joke between you and your friends of how pathetic he is. I've had this build for months secret, usually selling my items at the end worried of people stealing the build. Garen, Demacia, Justice. Break the mold, I will show my secrets of demolishing teams with Garen.

Reds= Greater Mark of Desolation x9 (1.66 Armor Penetration)
Yellow= Greater Seal of Evasion x9 (.75% Dodge)
Blue= Greater Glyph of Clerity x9 (-.005% Cooldowns reduction per level)
Quintessence= Greater Quintessence of Desolation x1 (3.33 Armor Penetration per level)
Quintessence= Greater Quintessence of Movement x2 (1.5% movement speed per level)
Statistics
Cooldown % at level 18 = +-8.62%
Armor Pen. = 18
Dodge= 6.8%
Movement= 3%

21/9/0.
Synchronizes with the runes. You're starting with 31 armor penetration, full damage. Early games you are going to rack endless kills up from the OP damage Garen has. This build was dominating before he got BUFFED, now even his Q hits like a brick with the 2.5 second silence and big movement speed buff. One major point to discuss is you have to learn to harass. Early game, a full spin should drop enemies to 25%-50% health. You are not killing someone early game in one spin just because you have brush advantage. You are in and out, when they get cocky with their halfish health, you can get them.
Yellow dodge runes are a must, with Ninja Tabi and defensive mastery, you are going to demolish dps classes.
Cooldown reduction is an absolute requirement on Garen. His skills are on somewhat medium length cooldowns (besides his shield), and having the cooldown reduction makes sure you are not just a one-trick pony, rolling in once and you're useless. With Youmoo's you will punish with silence, shield, and spam spin.

I've tried many items to start, none come close to the mighty REGROWTH PENDANT. You're starting with full armor pen, the Regrowth Pendant combined with your passive with have you laning like a beast. Believe me, you will kill the enemy champions multiple times, they will shop, and you should be able to farm at least 2000 gold still killing the enemies with only a regrowth pendant. This is when I learned just how OP garen is, thanks for buffing him too Riot.
You don't want to shop until you can at least get brutalizer and boots, giving you everything a Garen needs. By the time you get brutalizer, you will still be doing full damage even with their armor scaling.
Next comes giants belt/ruby crystal+chain mail, making you tanky, still with great damage.
The kills flow in, grab warmogs and your crit cloak or full atmas, and game should nearly be over already. If magic is giving you too much trouble, grab a Negatron cloak while building the rest of Youmoo's. Negatron can be built in to what you prefer, however normally with Garen's scaling armor/MR by leveling and his W, you should notice just how tanky you are by default. I RARELY need to buy a Negatron Cloak.
Youmoo's stats are yet better, giving you more armor pen to do ~full damage by the time you get it, with crit from this and atmas scaling his spin and auto attack. However, the MOST IMPORTANT part of Youmoo's is the passive. DO NOT trigger it before you spin or such. You should activate this when spin is on cooldown and you are going on an auto-attack rampage. You WILL be hitting FAST and critting for 500-1000.
I choose frozen mallet for the slow Garen needs, health boost (which translates into more Atma's damage) and little bit of damage. Your last items are completely up to you, however DO NOT get an Infinity Edge or Phantom Dancer. Black Cleaver, Last Whisper, Quicksilver Sash are most useful.

If you've already read this far in the build, you can see I have no care to grab flashy charts for you all. I am revealing my secrets to Garen, take it or leave it.

E,Q,E,W,E,R,Q,W,E,E,R,Q,Q,W,Q,R,W,W

OP spin early game raking in the kills, W scaling your armor/magic resist as you go to balance with the dps items, and Q hitting hard consistently throughout the game. Enough said.

Use the two most popular summoner spells that were designed for Garen.
Exhaust= DO NOT use this every single time you try to gank from brush. You can get plenty of kills without exhaust, but this will come in handy when using will GAREN-TEE the kill. Exhaust when in a battle with a carry 1v1, pop Youmoo's, done. Too handy on Garen to give up.
Ignite= Must have. Run around with Garen for a while and see just how unreliable Demacian Justice is. Ulti enemy, almost dead? Ignite. Win. Can also ulti+ignite and score easy 40% kills without having to over extend. Ignite can be used if baiting an enemy champion, lower their health, come back ulti+spin win. Ignite goes hand in hand with Garen with many mind-game techniques.
Ghost= You have run speed with your Q yo...
Flash= Does come in handy with some kills and suprise bush ganks, not to mention 99% of people carrying flash now.

In summary, yes this is a quickly written, non-flashy build, that may have burned your eyes reading. You will carry (nearly -_-) every team to victory. Fearless, murderous Garen will be the heart of your team and the fear of the entire enemy team, drawing focus to your +4000 health and +200 armor/+100 magic resist, or else you will brutally destroy teams in your path. Most OP champ I have played to date since beta, a real build coming from real gaming. Pwn your enemies endlessly, I've submitted my build in fear of theft. =)
I apologize for not putting some lengthy strategies, laning, fight tactics, etc. Perhaps I will put some videos in the future, despite that I rarely ever use mobafire as it is. Builds come from experience, not mobafire. Feel free to play around with the build, it should not be the same every game, as any player should know. Please post feedback, I Garen-Tee you will see a scary Garen revolution. If you don't want others taking this build or just want laughs, sell items before game ends and buy sorc boots+multiple archangel's staves.