Hello I'm Worthee and this is my basic champion guide for Garen. This guide is very basic, and is intended for low ELO, so you should look at more indepth guides if your Gold or higher. Maybe even if your silver or higher.

Pros:
Incredible Base damage with op scalings
Extremely Tanky
Good mobility
Incredible Sustain
Deals True Damage
Has a Super Execute
Manaless

Cons:
Easy to kite
No Dash
No CC
Low Mechanics

Just standard Flat Attack Damage Marks, Flat Armor Seals, Flat Magic Resist Glyphs, Flat Attack Damage Quints. These are my runes, and of course, may not be the best. Doesn't really matter, as it works fine for me.

Go 12/0/18, or 12/18/0. 12/0/18 is recommended for Tanking and being a bruiser, but 12/18/0 is also viable for giving the advantage during the laning phase. Get thunderlords decree tho, if you are going 12/18/0.

Q is your most used ability, great for chasing, escaping,_________,(geddit? Silence. lol.) farming, tower shredding, DAMAGE, etc. Great skill.

W is obviously maxed last, as ranking it up doesn't give as much of an advantage than ranking up all of his other abilities. However, this ability is literally AWESOME. It gives you and extra 30 armour and magic resist and protects you from ALOT of damage when activated. Great for tanking in teamfights, tower diving, and just all the good protective stuff.

E, aka Spin to Win. Spin to Win. Nuff Said. This ability is great with Black Cleaver, as it hits up to 10 times over 3 seconds at level 18. Instantly applies max stacks of Black Cleaver in an AOE. Also scales incredibly high, and has a relatively low cooldown.

R, The ultimate, aka DEMACIAAAAAA!!!!. Junglers use smite to delete monsters and creeps. Garen uses Demacian Justice to delete adcs and apcs and jglers and top laners and supports. OH wait. That's all the roles. Well it's true. This ability has a INSANE base damage and scales off the targets missing health, making it the strongest execute in the game. The passive also marks and enemy with the highest killing spree as the villain, and Garen will do 1% of the targets health as true damage when fighting the villain, and his ultimate will do TRUE DAMAGE. Judgement, with 10 attacks, will do 10% OF THE TARGETS MAX HEALTH AS TRUE DAMAGE. AND HIS ULT DOES TRUE DAMAGE. UNBLOCKABLE DAMAGE. It's like as Darkk Mane said. Playing Garen. 'I did it. I pressed all the buttons. All TWO of them.'

Normally start doran's shield and a health pot. If Vsing heavy ad like riven, consider cloth armor plus 4 heavy potions. As for Core items, Black Cleaver is essential for Garen, this item along can shred the enemy backline while giving you health and cooldown reduction so you can spam your abilities more.

Choose between Tabis or Treads depending on the enemy comp. Sunfire cape is a good item overall, but can be switched out for thornmails or other situational items. Steraks Gage give you health and damage, and a great passive in teamfights. Dead Man's Plate gives you health and armour, and a really great passive. Spirit Visage is of course for the Health and Magic resist and also the sweet passive of 20% bonus healing.

Situational items include Thornmail which is great against champions like tryndamere, or master yi. Combined with sunfire cape, you dont even need to attack. Just sit on them and watch them kill themselves, literally. QSS is great against malzahar, warwick, or yasuo. Champions that lock you up and rip you to shreds. Warmogs for health and passive, banshee can be swapped out for spirit visage (not recommended. Spirit visage gives better stats for garen and a more useful passive). Dead man's plate can be swapped out for Randiuns.

Early game, you should farm as much as possible as that will give you max armor and magic resist from your W. Farming is extremely easy from the immense eaveclear potential from your E. During levels 1-6 you can zone out your opponent using your Q, and also last hit with it.
From level 11+, you will become superstrong (assuming you aren't SUPER behind) and you can 1v1 almost any target. Remember to use your W when towerdiving, peeling, jungling, etc.

In teamfights, you are the tank, and should peel for your backline. Should the opportunity present itself, do not hesitate to q + r a high priority target. Also remember to spin to win, and it deals tons of damage to the enemy team. Try and hit as many targets as possible as it will give them max stacks for Black Cleaver making it extremely easy for your team to burst them down. Also, if the villain is a high priority squishy target, try to target them as you can literally just q + e then and they WILL die. Using Q on the enemy caster is also very helpful as you will silence them for a solid 1.5 seconds, more than enough time for your team to target them and kill them.

Overall, Garen is a very easy, yet strong, champion to play. With just 2 or 3 games, you should already understand most of the tactics when playing Garen, as he is rather a skilless champion. BUT HE IS OP.

Thanks for Reading, this is my very basic Garen guide with matchups. Hope this helps, i made this very quickly because i was bored so this will probably suck :/