Hi there! Welcome to my guide! This is my first full guide which I have actually put some effort into. It is an extension version of my first guide Triforce Garen (which I have finally updated yay)

Garen is not a mechanically difficult champion. This means that in order to succeed, you need a good understanding of the game.

Pros

Great sustain
Strong early game damage
Tankiness
Dueling
No mechanics

Cons

No mobility aside from movespeed gained from Decisive Strike
No outplay potential
Predictable

Perseverance

Garen's Passive is what makes his laning phase so strong. It gives him more health regen when not in combot, doubled when he is below 25% health. Lane minions do not stop this.

Decisive Strike

Garen's Decisive Strike deals a lot of damage at level 1 which is why you take it first. It is great for a AA Decisive Strike AA combo or simply a Decisive Strike. This silences the enemy for 1.5 seconds and breaks Garen out of slows and gives him 30% movespeed bonus. Decisive Strike can be used for charging or escaping.

Courage

Garen's Courage makes him extremely tanky. It gives him tenacity and reduced incoming damage. Use this for when an enemy is about to CC you to keep the CC as short as possible.

Judgment

Garen's E is his main damage and waveclear tool. When trading, try to let it only hit the enemy champion because it does 33% increased damage. It also reducing the armour of enemy champions who have been hit by four spins. E cannot be cancelled by anything other than stasis. A silence will just prevent you from reactivating it early.

Garen's R is the execute. This is used to finish off low health targets. It's passive marks the enemy champion with the most kills in the last 5 mins as the Villain. You do 1% of their max health as true damage and your R will do 100% true damage instead of magic damage. Usually the carry will get the Villain mark so in teamfights aim for them and finish them off quickly. This does not mean that you always go for the villain. Go for the main damage dealers and try to get all their attention on you so that your team can finish off the rest of the enemy team.

With the new runes this season, there haven't been any good runes for damage on Garen. Subsequently, Grasp of the Undying is what I think is the best choice right now. It gives Garen the extra health he needs.
Demolish is the only real choice here, Shield Bash and Font of Life aren't as effective. The health Garen gets from his items and runes will help destroy turrets, especially after the turret plating has fallen.
Bone Plating is the go to for genral tankiness as it helps you with trades in lane. If you are playing against a ranged matchup, go for Second Wind
Triumph gives Garen added safety to his tower dives. However, if you are playing against a team with a lot of CC, switching out Triumph for Legend: Tenacity may be the better option.
Coup de Grace allows Garen to easily take down low health targets with his R or a Q E combo.

For Offensive go Adaptive Force. The extra damage helps with trades early game when poking with Q
For Flex take Armour or Magic Resist depending on the lane opponent.
For Defensive pick up Health however, if the enemy laner is a poke champion or one with a strong all in (Pantheon, Teemo) take Armour or Magic Resist respectively.

The Black Cleaver synergises extremely well on Garen. His E reduces amour and in addition to the Black Cleaver passive will shred a lot of an opponent's armour. However, in winning matchups, Trinity Force is much stronger, especially if the enemy team does not have any super tanks such as Cho'Gath or Dr. Mundo.
Mercury's Treads are usually the go to item for Garen. It gives him the extra tenacity and MR. Ninja Tabis Ninja Tabis can also be bought if the opposing team has a lot of AD or AAing champions.
Dead Man's Plate gives Garen the more movespeed to chase down his targets. If you are completely steam rolling your lane though, you can push this back and instead, purchase a Youmuu's Ghostblade to further snowball you lead and close the game before the enemy team can deal with you.
Spirit's Visage Spirit's Visage lets Garen's P more healing letting Garen stay wherever he is for longer.

This build works best when their team is relatively immobile, not too many blinks or dashes.

I choose Phase Rush because it gives you the speed to catch up to enemies as you have no dashes. For the secondary tree, if you are confident,you go for Precision, using Triumph and Coup De Grace .

When hiding in a bush, open up with your Q , an Auto Attack, then E . This gets an easy proc of Phase Rush for chasing and gap closing. If necessary use R for the kill.

Stormrazor is excellent for that extra damage on your Q . Coupled with Infinity Edge , this greatly increases Garen's damage.

Picking up an early Hexdrinker is not a bad idea if the enemy team has two or more AP champions or there is a champion like Veigar who does large amounts of magic damage.

I'm too lazy to do a segment for every single possible matchup so I will only put the hard/common matchups here. For most other matchups, try to keep poking with Q and if you can without pushing the wave E .

Darius
Go Phase Rush and build Bami's Cinder. This allows your Q E combo to proc Phase Rush. Keep doing short trades until you have enough to all in.
Gnar
This is an extremely hard matchup. Depending on how he plays, either go Ninja Tabi's Ninja Tabis if he is autoattacking a lot or Mercury's Trreads if he uses Q . Play very defensive and ask for ganks. If he's bad, you should be able to get a few Q's off. This will lower his health a lot if he is in Mini-Gnar form. Trade when he is in Mega-Gnar form as he has much less range. When chasing him, use W to reduce the slow of his Q . Don't expect to win this.
Pantheon
Start Cloth Armor and 4 potions . When he is about to W , activate your W and E to reduce the stun duration and get easy damage. Follow up with an AA and a Q .
Teemo
This isn't actually that hard of a matchup. Play defensive until you get 1200g for a Spectre's Cowl or a Hexdrinker for an extra 100g. This will allow you to trade with Teemo easily. Use your Q to run at him and W when he is about to blind you. Keep trading like this and eventually you will be able to kill him or he will have to back.
If you want more matchups comment in the discussion and if I get enough requests for a particular champion I will add it.

Garen's early game is extremely strong so you want to trade as much as possible. Go for extended trades because your health regen from your passive will heal you back up ready for another trade. If you get the opposing laner low (30%), it opens them up to a Flash Q R for an easy kill. Try to poke with Q when your laner is about to go for farm. They will be stuck in their auto attack animation and will not be able to do anything about it. Against ranged opponents, hug your tower or try to bait out their mobility spell. If they use they mobility spell, use Q to run up to them, then use E . Use W when they try to CC you. If they have a slow, walk towards them and bait it, then use your Q to negate it.

Don't shove with E if the opponent is in lane. Unless you are extremely far ahead and can easily 1v2, sitting in front of the enemy tower is just asking for a gank. Garen has no dash or blinks making ganking him easy to pull off. Try to keep the wave in your half of the lane. If you do however find yourself near the enemy tower with the jungler coming up river, you have two options. Run or fight. 9/10 times it will be better just to back off using Q to get to safety. If you have the defence against both the jungler and laner, you should be able to fight them both and with post-6.

Garen can reliably solo Rift Herald from level 9. Use your W to reduce the knockback duration and damage from the initial charge. After that, spam your abilities until the Herald is dead.

If you know you can kill the enemy carries in a 5v5 without losing the rest of your team, go for it . However, this usually will not happen. Most of the time, you will be trying to frontline and tank as much as possible, silencing the AP carry or the tank. Try to use your E to reduce the armour of the enemy team as much as possible so that your AD Carry (or anyone else) can kill them quickly and easily.

1. Activate E and spam ctrl + 2 (taunt) to stand still while spinning.
2. Activate Q while casting E to get extra movement speed. Activating this then cancelling E will still let you get the Q off.
3. Activate R Deamcian Justice while casting E this will make killing enemy champions quicker.

Dark Harvest has seen to be extremely OP on many champions this patch (8.23). Howevet, Garen, is not that great at abusing it. His melee range and lack of gap closer make it extremely hard to make use of Dark Harvest. The only time Dark Harvest is effective is against another melee champion (preferably one that has no gap closer or mobility-think Malphite). Often at times, taking this keystone on Garen will feel like you have no keystone at all and are disadvantaging yourself in champ select.