Garen lives up to his lore. He is one of the most impressive champions in the game and perhaps the most powerful early game champion period.

He boasts incredible skill based output, a reliable execute, good mobility, a slow shed, a long silence, free defensive stats, no resources to manage for skills, and decent scaling.

For this build:

Advantages:
Great mobility
Constant Slow
Good damage output
Very Sturdy
Good initiate

Disadvantages:
Won't Carry
No tenacity (consider revising if against heavy CC)

Garen is a good off-tank/fighter

He offers a long silence and enough damage to be threatening to carries while being sturdy enough for the front lines of the fight.

He should be played as an initiator offensively and a protector defensively.

Garen is one of the few AD champions in the game which lacks a lunge ability. Due to this, you are heavy reliant on summoner skills, surprise (ganking), or teammates to get in melee range. Once in melee range, Garen must keep his target in melee range for the remainder of the fight or he'll get kited.

The frozen mallet is the perfect item to do just that.

Additionally, adding the mallet's slow will allow your team to focus your silenced target more readily and allow you to better shield your carry from their front line.

The mallet also provides health which synergizes well with the free armor and mr that courage provides and will provide additional AD once the Atma's is completed.

Since Garen's passive restores a heavy amount of HP over time, the Frozen Mallet takes precedence in the build order due to its other benefits.

As stated earlier, Garen relies heavily on summoner skills to close on certain opponents.

Flash is the single best skill to get on Garen as it allows you to flash and quickly silence and enemy before it can react. You can then spin to win (judgement) without the enemy being able to flash or lunge out of the ability's range.

Exhaust is my second choice summoner skill. With the addition of cripple it provides both a great defensive debuff to their output for an extended period and also shreds armor and slows which allows Garen's judgement to do massive damage effectively. Exhaust is also great to protect yourself against ganks and tower dives and also to shut down/ignore an enemy fighter long enough to focus down the carry.

I go 12/18/0 with Garen.

In the offense tree I grab critical strike chance (this improves judgement's damage as well as auto attacks), cripple, cdr, armor penetration, and Archaic Knowledge. AK is a hidden gem for Garen. The CDR is a given to get for him and for just one mastery point, the damage on your ultimate is drastically improved.

I put the rest of the points in the defense tree. The extra defense will provide more effective hit points to Garen and make his passive health regen more powerful. I like to grab nimbleness on Garen and complement it with a ninja tabi. I find that once the tabi is built the triggered move speed is on par with boots of swiftness but the tabi offers additional defense. It's really useful against AD champions, especially in the lane phase and pretty meh otherwise. In a blind pick, its a gamble. In a ranked game make a good guess at what you're up against and plan accordingly.

I almost always will get to the bush in my lane prior to choosing a skills. If I'm with a teammate with a cc I'll get judgement, if they don't I get decisive strike.

If I'm solo, I will wait in the bush. If one opponent face checks the bush, he'll find a Garen spinning on his face. If more than one shows up, they find a Garen running away, really fast like a little girl. If it's a 1v1 and they don't check the bust prior to the first last hit I get decisive strike and begin my farm.

Level two I take judgement or DS (which ever I didn't get at lvl 1)

Level 3 I get rank two judgement.

Level 4 I get courage.

I then begin maxing judgement whenever my ult isn't available. I max DS second and finally courage on levels 14,15,17,18.

I do this to lower the cool downs on the offense abilities and add to their damage.

Courage can be left at level 1 and still achieve the maximum passive benefit which is suggested.

Armor Pen marks and quints:
Since so much of Garens damage is physical and skill based, armor pen runes are an obvious choice.

MR glyphs and armor seals will complement the large health pool in the build and increase the value of the health regen passive.

I start off with on of two setups. I'll either go with boots and health potions against ranged or a doran's blade against melee.

Boots will allow you to get into melee range and deal damage and also avoid harassment. It also allows you to get to creeps faster to last hit.

A DB will allow you to go toe to toe against another fighter more readily and the additional life steal provides a bit more lane presence.

I then build either a ninja tabi (if against melee) or boots of swiftness against ap.

Next I build a brutalizer. This item's CDR will greatly improve the effectiveness of Garen.
While the ArmPen and AD will significanlty improve your abilities' damage.

I then begin on the frozen mallet, usually beginning with phage. Once the Mallet is completed I build an Atma's Impaler. Your damage will significantly improve upon completing this item.

I then upgrade the brutalizer to a ghostblade. This will help your damage and utility considerably since your skill based damage will begin to fall behind at this point and your passive move speed buffs aren't as powerful in team engagements.

I then build a force of nature. This has an impressive MR value, a movespeed buff to it, and great hp regen passive.

My final item is a Warmog's. This will complement your defensive stats as well as the FoN AND the atma's impaler. (You've probably already sold the dorans blade before this step but if not the Warmog's replaces the blade. (Sell the blade at any point if the sell value is enough to get you to larger benefit item [rather than returning later after 200g or going without it for a long time.])