Reasons why I like Garen

• He is extremely fun to Play
• He is the best Spinning-Swordfalling-DEMACIAA!-Crying-Tank-Knight
• He is very durable with good damage
• He is the only champ who has to go to toilet
• He doesn't have mana so you can spam out youre abilities

I would appreciate you to read the full guide before you start playing and before you downrate or write trolling commentaries and BTW I'm not english so please do not complain about my spelling or word usage.

Patch History (for the Guide and Garen)
In Patch v1.0.0.136 (Lulu Patch):
"Fixed a bug where Demacian Justice would not finish casting if Garen lost vision of the target during the cast"

In Patch v1.0.0.125 (Riven Patch):
"Decisive Strike attack damage ratio decreased to 1.4 from 1.5
Judgment
Damage per second reduced to 50/90/130/170/210 from 60/100/140/180/220
Bonus attack damage ratio increased to 1.4 from 1.2"

Latest Update:
2011/10/8 Reworked the masteries after the patch and added some things mentioned in the comments
2012/17/4 -Added Patch History
-Tweaked some things in the item section

Passive

Perseverance

Garen regenerates 0.5% of his maximum health per second after not receiving or dealing damage for 7 seconds. This ability makes you stay in lane so much longer and gives you much more farm.

Decisive Strike

Garen gains 15/20/25/30/35% movement speed for 4 seconds. His next melee attack within 6 seconds will deal 30/45/60/75/90(+1.5 per attack damage) physical damage and silence his target for 2.5 seconds. This is youre early game Harras ability because you get movement speed a does deal quite a bit of damage. Later it will be interesting because of the silence, you can just silence the enemies AP carry so he can't fire any spells. Another cool thing is that you can use the bonus movement speed for getting out of the base faster and for escaping.

Courage

(Passive): Garen's armor and magic resistance are permanently increased by 0.5 every time he kills an enemy unit (maximum 25) additional armor and magic resist). (Active): Garen places a defensive shield on himself, decreasing all damage taken by 20/24/28/32/36% for 3 seconds. This is your primary defensive ability. It will give you some pretty good armor and MR as well as let you absorb up to 36% of the damage at rank 5 wich is really good for tanking turrets.

Judgment

Garen rapidly spins his sword around his body for 3 seconds, removing all existing slows and reducing the duration of new slows by 50%, and dealing 30/50/70/90/110 (+1.2 per bonus Attack Damage) physical damage to nearby enemies every 0.5 seconds. Damage to minions is reduced by 50%. The bonus damage is further increased by Garen's critical strike chance. This is your main source of damage. It's an AOE spell and due to the fact you often will iniate in the mid of the enemy team you can just cast the spell and anybody will get damage. Another cool thing is that you can remove all slows so if someone casted Exhaust on you it can be removed.

Demacian Justice

: Garen brings down Demacian Justice on his opponent, dealing 175/350/525 magic damage plus 1 additional damage per 3.5/3/2.5 health his opponent is missing. This is your execution ability and it deals ridiculous amounts of damage. The only problem is it's short range so you sometimes will have to flash after or use the movement buff of youre Decisive Strike

First I take a Point in Decisive Strike taking 2 Points in Judgment after that. At level 4 you take one point in Courage and stack it up so you get 25 Armor and MR. 5th you take another Point in Decisive Strike for better Movement Speed Buff and more damage. Now just level-up youre abilities in this order: Demacian Justice> Judgment> Courage> Decisive Strike

For Garen i normally take standard tank runes, which gives him early and late game resistances and will grant you much more farm. But there are some other viable Options wich I will explain below

Greater Mark of Desolation: I take these because most parts of Garen's damage are physical and especially Late Game ArmorPen becomes a huge threat. Also many solo top players actually have armor runes wich can be countered by getting ArmorPen.
Greater Seal of Armor: These are great for early survive ability so you can stay in lane much longer and get more farm
Greater Glyph of Scaling Magic Resist: They give some great mid to late game Magic Resist so you don't have to build so much more
Greater Quintessence of Health: what can i say more, it gives you great early game power and durability

Other Options

Marks

You could take Greater Mark of Scaling Attack Damage, Greater Mark of Attack Damage for some damage but i don't think it really benefits so much or even a Greater Seal of Critical Chance wich will make your Judgment much stronger.

Seals

Since Dogde is taken out there are these options for Seals Greater Seal of Defense for a greater amount of armor but I think the early game bonus from Greater Seal of Armor is better

Glyphs

For Glyphs the best Option would be some Cooldown Reduction with Greater Glyph of Cooldown Reduction or Greater Glyph of Scaling Cooldown Reduction because due to the fact Garen has no Mana CDR is a really huge benefit

Quintessences

For Quints there are some good Options e.g. Greater Quintessence of Desolation for more Armor Penetration, Greater Quintessence of Attack Damage wich gives more damage. As suggested in a comment: here are Greater Quintessence of Critical Chance for increased damage on your E.But even unusual runes like 1 Gold/10 sec for support Garen, movement speed, CDR, armor or magic resist are viable for builds like Support Garen.

Defense Tree

I take Hardiness and Resistance for some early Defense and sustainability in combination with the Runes, aswell as some improved MR for late game. In tier two i max both Durability for Health and Vigor for increased Health Regeneration wich is huge in combination with your passive. I take the point in Veteran's Scars wich gives you a good amount of Health from the beginning, i skip out Indomitable and Evasion here because they are just rarely effective. Tier four with Siege Commander for defending towers Iniator for movement speed (pretty cool together with your Force of Nature) and Enlightenment for hardly needed CDR gives you a lot of the Utility you will need to succed. Last you go for Honor Guard because reduced damage is what you want and Juggernaut with CC reduction and EXTRA HEATLH.

Offense

Here just take 3 points in Brute Force for early harrasment and farming and Summoner's Wrath for improved Ignite. If you play with other Summoners you may want to take this point in either Summoner's Resolve or Summoner's Insight .

Utility

In Utility just take the point in Summoner's Insight (aswell as described above you may want to change this although i recommend always taking flash)

Starting Items

There are many viable starting Items:
Doran's Shield: It gives you some health, health reg and Armor but since you can't buy another pot it's not as good anymore and i would prefer....
Doran's Blade: The 10 attack damage are huge early game because it makes your harras so much stronger.
Cloth Armor + 5 Health Potions: this start will give you some good armor if you're going solo lane and the Health Pots will make you stay in lane much longer.
Boots of Speed: Starting with Boots will make your Harrasment much stronger and the three health Potions will help you too. THis is actually my favourite starting item because of the it also make you much less harrasable.
Regrowth Pendant: For support Garen this will be your favourite start item. It just adds a huge bonus to your passive and you can harras like *****
Ruby Crystal: I often see people starting with this one. It's not the best but i like to take it if I lane with a strong damage dealer, you also can build a quick Heart of Gold with it.

Boots

Boots of Swiftness: You normally don't want to take these because you are not going too roam around the map like Evelynn or Shaco
Berserker's Greaves: You can take these but since you're not going to play on Damage there are some better options
Boots of Swiftness: They are not bad but since his base movement speed has been increased they are not as good as before
Ionian Boots of Lucidity: Some more CDR is not bad and if you really want to spam you're abilities it's a good choice
Mercury's Treads: It's a perfect Option if you're facing a heavy CC or a Caster team. This item is my normal choice because with your Magic Resistence Runes you'll have enough MR for defending the average Mid Laner. BUT if the enemy casters get too strong you might want to go for some additional MR.
Ninja Tabi: A really cool item with the new effect. If you're laning a character like Riven who put out lots of normal damage, or if the enemy team is mostly physical with few CC these boots are really beneficial.
Sorcerer's Shoes: Never ever take these. You are not dealing magic damage so why taking Magic Penetration?

Magic Resistance

Force of Nature: A really good item wich i normally have in my core build, I gives a really high amount of MR, the bonus movement speed will also help you out and the health regeneration synergises very well with his passive Perseverance
Spirit Visage: It doesn't give you so much MR but the little bonus health and CDR are pretty cool
Quicksilver Sash: A good item especially with the active effect wich is like a free Cleanse
Hexdrinker: A very underrated item but for Garen it's really good, because of the attack damage, the passive effect is also really cool if you face a caster wich can just nuke you away (happens sometimes although you normally have MR) it will shield you for 300 damage
Maw of Malmortius: I did not actually tried this item out, but just from the effects I think it can fit very well if you're playing the role a of tanky DPS and bruiser. The shielding effect is more a kind of mid game thing but it might really pay of if you face the more nukey type of mage.

Armor

Randuin's Omen: Decent Amount of Armor, Health, Life Regeneration, CDR and a slow wich scales with Armor and MR. In other words: perfect item for Garen
Sunfire Cape: Armor and Health are huge for Garen and the passive effect is very good for youre Judgment because if you don't have any AD you can't kill a minion wave with one usage of you're E but with some burning Sunfires this is possible
Thornmail: Also a good item especially if you're facing someone who deals really huge physical damage like Tryndamere or Ashe but normally you will get enough armor with your other items
Atma's Impaler: This item not just good for the armor but if you get something like a Warmog's Armor it also gives you a good amount of Attack Damage and increases the damage of your Judgment

Offense Items

Frozen Mallet: It is a good item due to it's slowing effect and health but since you're not playing on AD you normally won't buy it
Last Whisper: If you want to deal some good damage but face another high armor guy it will really help you but normally you will deal enough damage even without this. Also as described in the Runes section ArmorPen will really help you out Late Game.
The Bloodthirster: Best item for getting some Attack Damage but like with the items above you're not playing on AD
The Brutalizer: Pretty good item with some attack damage, CDR and Armor Penetration and you can also turn it into a Youmuu's Ghostblade wich will increase the bonuses
Cloak and Dagger: In my Opinion this is not a bad item but since you can easily get the Tenacity from you Mercury's Treads and Attack Speed as well as CrC are not the favourite Stats for Garen I'd skip this item.
Ionic Spark: A really mediocre down to bad item for Garen but the thunders are nice :)

Situational Items

Warmog's Armor: This is an item i really like to take because you will tank so much better after you got this because 1.4k Health are huge. It also synergises well with Atma's Impaler
Guardian Angel: If you're doing pretty well and keep getting focused (unusual) this can help you not dying so often. It also gives a small amount of Armor and MR
Aegis of the Legion: If you're playing a more support style Garen this is a good item, because it will give your team some good bonuses
Leviathan: I wouldn't recommend this because it gives you much less health than Warmog's Armor and you normally won't get the kills

Supportive Items

I want to get this section on when I'll start working on this guide again because I'm thinking of doing a support Garen Guide in addition. Therefor here are some Options I didn't include above but are still more or less viable (without comment):
Heart of Gold Locket of the Iron Solari Kindlegem Zeke's Herald

My Normal Build

This Build gives you all you want: Health, Armor, MR, some CDR and some good Attack Damage
but one thing you should know is, that you shouldn't just always buy these items. You should always base youre build on the enemy team.

Spells to take

Flash: In my Opinion the best summoner spell, it's great for escaping, chasing enemies, stealing Baron/Dragon or postitioning right.
Ignite: Ignite is great for getting down enemies even if they run away. Especially early game it's great, because you can get easy kill with it
Ghost: This is great for escaping or chasing enemies but I would take flash over Ghost.
Exhaust: If you think you are going to chase many enemies down in the game it's also good if you're facing a strong AD carry
Cleanse: If you're playing ranked and recognize that the enemy team is strong on CC you could take this but In my Opinion it would be better to buy a Quicksilver Sash
Teleport: This can be really good if you're doing a solo lane. Because so you can go back, buy and teleport back to lane
Smite: If you're jungling take this but you normally won't do it.

Spells not to take

Clarity: Are you dumb? You don't have mana
Rally: This spell is completely useless no real effect, high cooldown, why take it?
Revive: Every 5 minutes you can skip the death timer. There are so much good spells wich grant you kills and around 3 more minutes of not being dead isn't worth it