Preferred Summoner Spells

• This is one of my favorite summoner spells. It's applicability to every game is unbelievable. Not only will it secure kills when they run fast than you or are in turret range, but it can cut heals in half like Maximum Dosage or Meditate, or even straight up Heal! When in doubt, always Ignite!
• Flash is by far the most useful and legit spell in the game with it's practicality even better than Ignite for every game. It's used for getting away, chasing, juking, etc. Regardless of the circumstances, I always use Flash.

Viable Summoner Spells

• Although I almost hardly ever use Ghost anymore, it's still a very viable and useful skill for Garen. I prefer sticking with Ignite on "D" and Flash on "F" and using Decisive Strike as a substitute for Ghost, it would still work and help you in chases.
• Again, I almost hardly use Exhaust anymore but it still works for Garen. Instead of Exhaust I just use Judgment instead, but it's entirely up to you depending on your needs.
• I'm not a big fan of Heal, and those in the higher skill ranges tend to frown upon it, but if you're just starting or have health issues, then go for it.

Any Other Summoner Spell

• I wouldn't recommend any other summoner spells for Garen, mostly because they don't make any sense. Self-explanatory spells like Clarity and Promote will not work with Garen. Even soloing top, Teleport is a bad option because Garen simply doesn't need it, and he will function better without it.

DPS Runes
These runes are completely offensive, lacking any resists and is for pure damage.

• Greater Quintessence of Critical Damage: This is what I use mostly for DPS Garen, with a whopping 4.5% crit damage each, this is what fuels Garen's damage.
  OR
• Greater Quintessence of Movement Speed: You can also use these for a mobility bonus of 4.5% movement speed, but I wouldn't recommend it as they are 2k each.
  
  
• Greater Mark of Critical Damage: I usually get 6 of these, which also fuels most of Garen's damage.
  
  
• Greater Mark of Critical Chance: Along with the 6 Crit DMG marks, I grab 3 of these to help Garen reach 100% crit chance at his full build.
  
  
• Greater Glyph of Critical Damage: I grab 9 of these to make Garen even more powerful, and Garen's natural tankiness and Courage makes up for the MR i could be getting.
  
  
• Greater Seal of Critical Damage: Same as the marks, I get 6 of these to add damage.
  
  
• Greater Seal of Critical Chance: Same as the OTHER marks, I have 3 to reach 100% crit.

Tankier Runes
These runes are a mix of offensive and defensive runes, not providing quite as much damage as the DPS, but this focuses more on durability.

• greater quintessence of desolation: To make up for the lost of Yoummuu's Ghostblade in my build, I grab 3 aPen quints as well as 9 marks.
  
  
• greater mark of desolation: These combined with the quints make up a large amount of aPen, which is needed to substitute the loss of the Ghostblade.
  
  
• Greater Seal of Armor: Standard flat armor runes that are cheap, practical, and almost every summoner has a set.
  
  
• Greater Glyph of Scaling Magic Resist: Because most AP characters can't do decent damage early game, I grab MR per level for a little later game durability.

Perseverance
Garen regenerates 0.5% of is maximum health after not receiving damage for 7 seconds.
This skill is what defines Garen's laning. Without this passive, he would be nowhere near as useful as he is now. Instead of recalling and missing out on XP and gold, you can farm behind minion waves to restore health.

Decisive Strike
Garen gains movement speed for 4 seconds, and his next basic attack will deal extra damage and silence the target for 2.5 seconds.
You use this skill exactly like it's named for. You can either silence the AP Caster's ult like Nunu & Willump or Malzahar, or use it to reset your attack timer to take down turrets faster. It's all up to you, and it's a great chasing/fleeing skill.

Courage
Active: Garen ignores a significant percentage of damage for a few seconds. Passive: each minion kill grants +0.5 armor and magic resist.
This skill is the most useful when tanking, and also provides a fair amount of durability. Use it in combination with Judgment to really dish out damage while taking little to none.

Judgment
Garen spins for 3 seconds, dealing damage around him while removing any slows and lowering the duration of any slows while in Judgment. Critical chances apply.
This ability... This is what makes Garen real. This is why you need to build 100% crit chance and 284% crit damage. This AoE is so powerful in teamfights, chasing, running away, it's applications are endless. Judgment is the skill that is used in all your combos, and without this skill Garen would be useless.

Demacian Justice
Deals magic damage to the target based on how many hit points they are missing.
Demacian Justice has many uses, including executions, KS'ing (kill securing), intimidation, etc. It is most commonly use by unskilled Garens stealing kills, but in the right hands it is one of the most powerful skills in the game. Use Demacian Justice to wipe out enemies with large health-pools like Cho'Gath and Dr. Mundo.

STARTING ITEMS

These are items you should buy with your starting amount of gold (475).

Doran's Shield
This is a very viable first item, providing armor, health, and health regen, this is one of my favored choices when playing Garen.
Doran's Blade
This is also a very viable first item, providing some nice AD, life steal, and a decent amount of health. This is a very viable choice to consider buying if you are laning with someone tankier than you, or you're against squishies.
Boots Of Speed + x3 HP Pots
This isn't necessarily needed, but if you know that their ranged AD Vayne is going bot along with support Sona, it might be wise to invest in speed and healing.
sight wardBoots Of Speed + Sight Ward
If you're soloing top, this can be a wise investment to prevent ganks from the enemyy jungler while keeping enough movement speed to run away/chase.

CORE ITEMS

These are your core items that must be included no matter what.

DPS GAREN

Infinity Edge
This one is a no-brainer. If you don't have this your runes are worthless and you can't deal any significant damage.
Youmuu's Ghostblade
This item was practically built for Garen. It comes with everything he needs; Armor penetration, critical strike, attack speed, movement speed, and attack damage.
Phantom Dancer
This item gives Garen his full critical strike while also giving him his required mobility and attack speed. You simply cannot skip this item.

TANK GAREN

Warmog's Armor
This item is crucial to Tank Garen's build. No matter what their team is building, health resists all types of damage (kind of).
Atma's Impaler
This item not only makes Garen durable, but in combination with Warmog's Armor, it gives him the damage he needs to function.
Guardian Angel
This item gives Garen durability in the form of both magic resistance AND armor. It's passive can save you from a team fight, which can also lead to you saving other's lives. It's an all around good item, because it gives both MR and armor.

SITUATIONAL ITEMS

These are items you should get depending on the team compositions and the level gap between players.

*NOTE:
As DPS Garen, you cannot change any items in order to keep the maximum damage output. Of course you may modify it depending on the circumstances, but to keep the raw damage output you must follow the cheat sheet exactly.

BALANCED ENEMY TEAM

Randuin's Omen
If the team is well balanced, their AD carry should be extremely dangerous. Getting this item for the active can also help in team fights, chases, and escapes.
Boots Of Swiftness
These boots give Garen the mobility he needs in team fights and when chasing or running away. CC Shouldn't be too much of a problem, and if so these will most likely get you out of their range before too long.

CROWD CONTROL HEAVY EMEMY TEAM

Mercury's Treads
Yup! This is exactly what you expected, right? Well you can either save an inventory slot, or buy Boots of Swiftness and get a Cloak and Dagger too.
Cloak & Dagger
This item is perfect for Garen if their team has a decent amount of crowd control. It gives him critical strike, tenacity, attack speed, and all at a relatively low price. This is a wise investment, just remember that tenacity items do not stack,
Quicksilver Sash
With an active the equivalent of Cleanse at a fraction of the cooldown, this is a wise investment as it can save you from people like Malzahar and Warwick's ultimates. It also gives a decent bit of magic resistance.

TANK HEAVY ENEMY TEAM

Last Whisper
I wouldn't entirely recommend this item, but it can prove to be quite useful if they are stacking armor.
Madred's Bloodrazor
This item will come especially in handy when there are very large hp tanks such as Cho'Gath and Dr. Mundo as the bloodrazors deal 4% of their max health per hit. It also gives a fair amount of attack speed, armor, and attack damage, which is all quite useful.

AD HEAVY ENEMY TEAM

Thornmail
I hardly ever build this item, so use this only if they are targeting you and you can barely survive 1v1s. Luckily it's relatively cheap at 2000 gold and give the highest amount of armor in the game.
Randuin's Omen
Yup, this item again. It's passive and active abilities can really mess an AD team up, giving you the advantage. Even though it costs quite a bit, it give quite a bit of stats back so you get what you pay for.

AP HEAVY ENEMY TEAM

Force Of Nature
Giving the highest amount of magic resistance in the game, and also utilizing mass HP regen and movement speed, this item will block out a lot of damage and help with your mobility to better position yourself in team fights.
Wit's End
Some say the best defense is a good offense. Well, this item gives you plenty of both. Fueling Garen's need for attack speed, it also gives him more durability the longer he's in the fight.

Decisive Strike and Auto-Attacks

When playing Garen, always remember that your Q, Decisive Strike resets your attack timer and increases your movement speed. If you don't know what the attack timer is, I'll explain now.

Here is how it works. Let's say you attack once per second. You just attacked and are .8 seconds toward your next attack. You activate Siphoning Strike, or Twin Bite, or some other ability like Decisive Strike. The moment you activate the ability, it resets the auto-attack timer, and it attacks immediately.

With this in mind, the best time to activate "on next hit" abilities is right after you just swung. You get a free attack with only milliseconds wasted. Seeing how Decisive Strike works on turret too (aside form the silencing), you can use that to your advantage to burn turrets quicker. Also keep that in mind when 1v1'ing people at close range, or when fighting people with channeled ultimates such as Fiddlesticks' Crowstorm as it can interrupt it and often save your team's lives.

Turret Diving

Garen also excels at turret diving. Turret diving is attacking a champion underneath their own turret. This is extremely hard because the turret does balls loads of damage, and it has quite a far range. Garen can successfully poke until the point where he can pull a full combo on them.

Upon bringing your enemy down to about 1/3 to 1/2 health, you're ready to turret dive. Make sure you have your W, Courage off cooldown and have at least 2/3 HP.
When you're ready to engage, start with a basic attack into Decisive Strike. Then proceed to Judgment. Make sure to use Courage upon taking the third turret hit, as turrets progressively deal more damage to the same target. If the target is low enough, ultimate them with Demacian Justice and then Ignite them if they survived. If you need, Flash or Ghost away. If you are using Exhaust, make sure to exhaust them directly after using Decisive Strike.

However, just because Garen is so durable, does NOT mean he is invincible. Always remember to last hit creeps, don't over extend, and call mias. Have some decent map awareness and always have an escape plan for ganks.

What to do if you're getting chased

Garen succeeds at chasing and running away. Hopefully, by this point of the ganking phase you should have finished your boots and possibly have another speed item like Phantom Dancer or Force of Nature. Regardless of your items, run either to the nearest champion or the nearest turret. Make sure to use Judgment if they get too close or they slow you. If you see one of them about to use a high damage spell like Paranoia or you see an "on next hit" ability charge, make sure to be generous with Courage and Decisive Strike whenever you can. If it helps, you can turn around and whack someone with Decisive Strike to keep them from doing further damage.

If they are chasing you through the jungle, make sure to utilize the brush and be as unpredictable as possible. Most people don't expect a direct 180 degree juke when running into a brush, and this can often confuse your opponents. Running further into their jungle could end up two ways; 1.) you completely discombobulating your enemies and escaping or 2.) you completely messing up and turret trapping yourself and die.

Well, this is the end of my Tank/DPS build for Garen. I hope you were able to understand and absorb all the information and my reasoning, I'd appreciate it if you'd rate and comment telling me what you think! Thanks for your time :D