Welcome,
This is my Critstorm Garen guide. Critstorm Garen is focused around having a 90% critical strike chance with increased critical strike damage cap.

Why play Critstorm Garen?
Because the burst of damage is rediculous. His (Q) Decisive Strike and his (E) Judgement currently benefit from Critical Strikes. Judgement, aka Bladestorm is now a Critstorm.

I run a 17 - 13 - 0 Masteries, maxing out Attack Damage and Critical Damage.

3 Points in Resistance and Hardiness for more Tankyness because you not building Armour and the total percentage of your Armour and Magic Resist will increase even further with Garens passive benefits from his (W) Courage. Also picking up Durability and Veteran Scars for more Health and Vigor for health regen.

I run the following Runes:
Greater Mark of Furor,
Greater Seal of Furor
Greater Glyph of Furor,
and Greater Quintessence of Furor,
These are taken for maximum increased Critical Strike Damage Cap, this stacks with your Lethality mastery and your Infinity Edge unique passive bringing your Critical Strikes to 305.69% increased damage.

Skills:
Perseverance (Passive) - Garen regenerates a percentage of his maximum health per second after not receiving damage for a duration.

Decisive Strike (Q) - Garen removes all slows on him and gains increased movement speed for the duration. His next melee attack will deal additional physical damage and silence his target for a duration.

Courage (W) - (Passive): Garen's armor and magic resistance are permanently increased by a percentage. - (Active): Garen places a defensive shield on himself, decreasing all damage taken and reducing the duration of crowd control by a percentage.

Judgement (E) - This is a heavy damage AOE that does not channel so it can be interrupted, and removes unit collision while active. Benefits from Critical Strikes.

Demacian Justice (R) - This is used for executing Champions on 1/4 Health.

Summoner Spells:
Ghost - Great for chasing down low Health Champions with your Ultimate or chasing fast Champions like Master Yi.

Flash - Always take this as it is a great escaping skill and kill secure skill.

Ignite - No need for and Execute Skill, Garen already has one.

Clarity - Useless as Garen has no manapool.

Clairvoyance - Does not have any skills to make real benefit of this, its more for support characters for a temporary ward.

Cleanse - Can be good, but Garens (Q) breaks all slows.

Heal - Useful, but not really needed because of Garens Passive.

Surge - Not a very good skill as Garen is not an Auto-Attack Champion.

Teleport - Very useful, can be used for ganking other lanes quickly or used to get back to lane faster.

Revive - You should never take this on any Champion as you should not plan on dying.

Smite - Useless as Garen does not jungle.

Promote - Good for pushing towers but thats all.

Trinity Force,
This here is your way to 1 shot adc's and take massive chucks of HP off Tanks and Bruisers.
Trinity Force is for Garens (Q) Decisive Strike.

Decisive Strike: Garen removes all slows on him and gains 35% movement speed for 1.5 / 2.25 / 3 / 3.75 / 4.5 seconds. His next melee attack within 4.5 seconds will deal 30 / 55 / 80 / 105 / 130 (+1.4 per attack damage) physical damage and silence his target for 2.5 seconds. This attack can critically strike.

It is a 1.4 AD scale, so heres the number crunch I am about to show you as to how much damage you can make it do if the target had no armour.
Indicates Attack Damage, Indicates Critical Strike Damage Modifier, Indicates Trinity Force on hit Modifier
Garen at level 18 will have 445.5 Attack Damage and a 90% Critical Strike chance.
So his (Q) Decisive Strike non crit does, 130 (1.4x445.5) = 753.7 Attack Damage.
So his (Q) Decisive Strike Critical Strike does, 130 (1.4x445.5) x 305.69% = 2303.98 Attack Damage.
So his (Q) Decisive Strike Critical Strike with Trinity Force Modifier does, 130 (1.4x445.5) x 305.69% x 150% = 3455.97 Attack Damage.