I know: this guide ain't really special, so it won't get popular (I think so). I just made it for those, who MAY have problems with Garen - this is the very basics, the Garen is based.
(P.S.: This guide is based on Phreak's build in Garen Champion spotlight)

The main mistake I saw sometimes, which is made by Garen players - Garen isn't a DPS-style-champion.
He doesn't need a lot of low damage strikes - he must deal a lot of damage in a few strikes.
Garen's main idea - is to get fed and carry the game. He's not a full tank, and he's a full DPS.
He is the very everything, what you call an OFFTANK.

With Regrowth Pendant and Perseverance passive, Garen gets a lot of HP Regen, which allows him to stay in the lane for quite a lot of time.
Critical Strike from Avarice boosts the damage on his E, which decimates our foes.
Armor Penetration is simply nice.
The main problem we get with such order - is squishiness early game. But late game we get balance resists - ~200 each, and ~4000 health, which will make enemies simply fear us.
Also we get Atma's, takes damage from Warmog's and Randuin's. We get ~250 AD lategame.

Garen must get kills early, so we take Ignite in case our enemies escape Demacian Judgment.
Ghost is simply a nice spell - it's a nice escape mechanism, a chasing mechanism, and so on.
Also can take:
Flash
Cleanse
Teleport
Fortify

Desicive Strike is getting maxed out first because of the speed buff, awesome AD Ratio, and a killing nature for AP Carries, which must not get fed early game, so they will get killed in lategame, cause of which will be no farm early.
Judgment is next, because... Just because... Oh, c'mon, it's GAREN'S JUDGMENT. It's an awesome damage-dealing ability, which is the main source of Garen's damage.
Courage gives us Garen's durability - you can tank a tower, tank a carry - you can tank everything. (Almost everything)
Demacian Justice is Garen's Ultimate.
It's works awesome for KS'es, and in that case, when your enemy is low and runs away. that's when you ultimate and score a kill.

Armor Penetration Marks and Quints - for higher damage throughout the game, and these runes work well with Youmuu's - we get 51 Armor Penetration midgame, and have it for the entire match. This ain't much, but with Garen's durability - it's simply awesome.
Dodge Seals - we get enough armor with items and W, so Dodge works just fine.
Cooldown Glyphs - Garen has no mana, meaning cooldowns mean pretty much for him.
(P.S.: Health Quints work too)

A standard DPS masteries setup.
We get enough defense with items, so we don't need it from masteries.

1. Find yourself a target (Ideally - a mage, or a AD-based champion, which deals damage with spells. For instance: Pantheon, Morgana, Veigar, Maokai, Rumble e.t.c.), run to that victim and silence him with Desicive Strike.
2. Shout DEMACIAAAA! (Turn your W on, reducing the damage you take)
3. Start a Danse Macabre (Press E)
4. Kill something.
5. In case somebody runs away - turn your Youmuu's on, Desicive Strike too, run to the non-killed ones, and execute them (Use R)

Ok, that's it.
This guide is made more for myself, 'cause I can sometimes forget, what i build :D
Credits go to:
Me, the Mighty GameOveRus (Ego OVER 9000)
Phreak - for making a champion spotlight for Garen, giving me the basic item and rune build, which I invented further and ended in this guide.
All my friends - I love ya all :D
Thank you!
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"To the Fields of Justice!" (c) Garen.