HI! I'm TAboss and my favourite champion is Garen . This is my first guide so excuse me if it isn't very good . Why I like Garen ?? Look on his Pros/Cons below .
When I play with my friends i usually pick him on solo top and very often we win . While I'm making this guide I have 24 lvl . I worked hard to make this guide the best I could . I'll improve it .

Pros

+ no mana
+ strong tank
+ very mobile champ
+ nice healing passive Perseverance
+ silence , good one hit damage and speed with "Q" Decisive Strike
+ good passive and active "W" skill Courage
+ good farm and harass with "E" Judgment
+ OP ultimate to kill secure ^^ Demacian Justice

Cons

- no corowd control need good jungler with CC
- weak against some champs for example Singed
- long cooldown in early game

My runes are only an example too . I use them and I am satisfied . I suggest to create your own Runes.

I use Flash to escape when ganked and Ignite to harass before Demacian Justice

Passive Perseverance gives nice heal for garen when he isn't attacked

"Q" skill Decisive Strike is good because it gives speed and it reduce CC on Garen. It also scilences enemy and inflicts very strong one hit damage.

"W" skill Courage has cool passive. It increases armor and magic resist by 20%. And active give you shield which increases armor and magic resist by 30% for 2-6 seconds.

"E" skill Judgment is very usefull for farming and harassing enemies

Ultimate Demacian Justice hardcore skill which deal bonus damage of enemy missing HP. Ideal for kill securing ^^

Doran's Shield 475 very good opening item for tank

Boots of Swiftness 1000 it provide very needed mobility

Warmog's Armor 2830 primary item for Garen

Atma's Impaler 2300 this item will provide you damage from your max HP and armor

Sunfire Aegis 2650 it gives armor , HP and helps in farming minions

Spirit Visage 2200 it gives magic resist , HP and cooldown reduction

Runic Bulwark 2950 this item makes your team more survivable ( if your support isn't building it you should buy it )


optional items:

Executioner's Calling 1900 good item against healer

Infinity Edge 3800 if you don't have to build Runic Bulwark I recommend this item

Black Cleaver 2865 if opponent team is tanky I recommend this item

Frozen Mallet 3300 it is exchangeable item with Infinity Edge

Mercurial Scimitar 3700 if you need AD against AP opponent

In teamfights Garen should initiate that means he should start fight with combo Decisive Strike and Judgment then use Courage . Your teammates should start as fast as they can after your initiation . You should focus the most fed enemy dealing the greates damage. When you spot enemy with low HP chase him and kill using Decisive Strike, Demacian Justice preceded by Ignite to make sure that Demacian Justice will kill the enemy. Don't waste your ultimate when you have time and it is possible to kill enemy with Decisive Strike

Farming with Garen is easy. You should attack minions which you can kill by one hit (last hit). Also Judgment is good to farm groups of minions. Sunfire Aegis is very halpfull in farming. When you want to be sure that you will kill big minion use Decisive Strike.

I've played a lot of games as Garen and in my opinion he is better against some champions and worse against others. So to help you I'll show against who Garen is good in my opinion.

Strong against:
Fiora
Weak Against:
Singed