Garen is one of the most dominating early game heroes, and one of the least item dependent heroes due to his amazing damage output with Judgment and good utility with all his powers. He can build almost any items and do well, but I think to fit into a team well (and it is a team game) he is limited in the way he can be built. This guide is for the Garens out there who end a game 15/5/15 and lose and don't understand why. I'm going to assume you know his basic powers, how the game functions, and I'm going to be short and sweet.

Things I want to stress in this guide

Garen is best as a Melee Caster - not an Auto Attacker.
Garen is best as an Offtank - not the main Tank.
Garen should NOT solo lane.
Garen is best as an Anti-Carry - not the main Carry.

Garen is a delicate balance of offense and defense. You need enough offense to kill the carry, and enough defense to get to them and to make it out from under enemy turrets.

Mark of Desolation - helps all your offensive abilities

Greater Seal of Armor - helps with early game laning and turret diving.

Greater Glyph of Scaling Magic Resist - With my build you're not getting MR for a while, so you're depending on these and your passive from Courage to get you by into mid game. I know Greater Glyph of Scaling Cooldown Reduction or Greater Glyph of Cooldown Reduction are typical, but we're benefiting from Masteries/boots for CDR, and need some MR.

Greater Quintessence of Movement Speed - Garen typically gets Boots of Swiftness, I am maximizing speed through Quints and masteries, so that we can get Ionian Boots of Lucidity to take advantage of Garen's cooldown only gameplay.

I realize 9/0/21 seems odd at first, but this build maximizes some things Garen really needs, notably Speed and CDR.

Exhaust is necessary as part of your Anti-Carry role, it helps you stick to your target and make sure they feel some Demacian Justice.

Teleport could easily be traded for Ghost, seeing how important speed is for Garen. However, with well placed wards or friendly minions he can help Gank or protect Mid or Top very quickly, and turn an otherwise successful enemy Gank into a surprise double kill when you show up behind them and they thought you were far, far away.

In your Offtank/Anti-Carry role, you must strike a delicate balance between offense and defense. You need survivability, because you will get some focus when you run straight for a Carry. You need to be able to live to do your damage, and you need enough offense that you can be pretty sure Q->E->R will finish them off.

I like Boots and 3 Health Potion because Garen's natural damage is some of the best early game, the speed makes sure you can stick to your opponent, and the Health Potion combined with your Perseverance let you be more aggressive.

The Sunfire Aegis is both offense and defense and a good early item, as the health and armor make turret diving for a kill viable (though not necessarily recommended), while still giving you some damage output.

The Infinity Edge is Garen's ideal item, as it boosts your damage output considerably, without giving you things you don't need (Attack speed, specifically). Any item that gives attack speed is not ideal for Garen. His non-ult damage output comes from Judgment, and attack speed is a useless stat while he's spinning.

From these 3 core items: Ionian Boots of Lucidity, Sunfire Aegis, Infinity Edge - I like to assess how the game is going to determine the order of my next 3.
Lot of AP damage -> Force of Nature
Lot of turret diving/physical -> Warmog's Armor
Not quite killing that Carry or if you're dominating considerably -> Atma's Impaler.

Don't be afraid of a late game Warmog's Armor with Garen. His late game farm is amazing and he collects assists and kills like it's his job (and it is). Also don't be afraid to grab item components like Negatron Cloak, Giant's Belt, Cloak of Agility without building the full item. You need to assess the situation and what you need more of (offense/defense).

I suspect someone will say I should get a Frozen Mallet instead of a Warmog's Armor or some other item. I don't like Frozen Mallet, as it's not procing on Judgment and he doesn't auto attack fast enough to make good use of it (and he shouldn't have attack speed!)

I also think Last Whisper is a good situational items for a carry with high armor, but that isn't a situation you often see.

So at the beginning I said this guide was for Garens who went 15/5/15 and lost and I want to defend some of my points and choices I've made so far.

Garen is best as a Melee Caster - not an Auto Attacker.
I see a lot of Garen's with Youmuu's Ghostblade or Phantom Dancer and I understand their appeal. But the attack speed portion of those items are wasted on him, and it is in essence money wasted towards a stat that doesn't matter that much for him. Also with these items you need more offensive items to get the damage output you need, making you squishier, and causing you to die too fast in team battles, and not get out all the damage you're possible of doing. If you do well with these items, you will tend to dominate early/mid game and in gank situations but fall off in usefulness in team battles, as you'll die too quickly (and often play too aggressively due to early game dominance)

It's better to get items that have synergy with Garen's melee "casting", so pure damage, crit chance. I think Infinity Edge and Atma's Impaler have the best bang for their buck, so to speak, with Last Whisper also being a viable item.

Garen is best as an Offtank - not the main Tank.
You do not have enough CC or Damage as Tank Garen to cause a smart team to attack you late game. A smart team will just ignore you. Due to Demacian Justice and your tankyness, you will likely still end the game with a great K/D/A ratio, but your kill(s) will happen while your entire team gets slaughtered, because there is no point in bothering with you. The illusion of the greatness of Tank Garen comes from bad teams focusing him late game when they should not attack him.

Garen should NOT solo lane.
Due to Perseverance, he CAN Solo lane, and I see it all the time. But he does not need the experience or the gold nearly as much as other Heroes. Garen is less item dependent than almost ANY hero in the game. He has naturally high damage output and strong defense/escape powers (speed boost/shield/slow removal) and health regen. Combined with his high speed and either Ghost or Teleport, he can typically help mid or top when needed, making him a very useful and mobile champion in the laning phase. He is also very good at getting early kills in bottom lane due to his high early damage output.

Garen is best as an Anti-Carry - not the main Carry.
He does not have enough sudden damage and consistent output to be a true carry. His powers need time to unfold, he needs time to get there, time to silence, time to spin, time to Ult (his ult is based off of someone already being hurt, if you can't live to that point, you aren't much use). He can completely take a carry out of a team fight, but not if he dies too quickly late game, and not if he can't do the damage he needs to do.

AND for the love of Demacia don't spin the minions!
You WILL almost always get some early kills in bottom lane with Garen if you have a competent laning partner IF you last hit. Exhaust and your powers makes it unlikely they'll be able to escape to their turret, or that you'll die under it. Only go for a minion spin if you need to push a bit so you can B and get an item.

How many times have I said this? I can't say it enough. Garen needs a good balance of offense and defense to really shine. Enough offense to make you scary and not safe to ignore, but enough defense to live to do your damage and to make a team question if they should target you. My build is fairly defensive, but it's because I have found that just those few offensive items are enough for you to nuke a carry as quickly as needed and enough defense to survive all the Focus you're going to draw from nuking that carry.