Introduction
The goal of this guide is to give a build for Garen that is both tanky and dpsy. Garen is useless as a pure tank since he only has one skill that CCs, and no way to hold the attention of your opponents. Garen as a pure DPS is also useless; without some tank runes/gear/talents Courage won’t keep you alive in team fights. Judgment requires you to be in the middle of all your opponents to do good damage – this makes you a target.
Because of these things your goal is to find a good balance between tank and DPS. It’s a fine line to walk – hopefully this guild will help you out!
It is important to keep in mind you aren't a tank with this build. You also aren't Ashe with 2xPD, IE, and a BT - you're not gonna kill people with two right clicks. What you are is a carry's worst nightmare. You're gonna run up, silence, spin, and lawl as they try to burn you down.
Note #1: most of the content of this guide will be found in the cheat sheets at the top, the subsequent sections are merely to explain why I made the choices that I made, with a brief explanation of some tactics at the end.
Note #2: This is my first guide, I've focused mainly on content rather then presentation. So if you down vote this guide, please leave a comment so I know what to fix.
Pros / Cons
Pros
-Probable the best lvl 1-3 DPS in the game
-Can be very tank and have very strong DPS at the same time
-Extremely fast movement
-No mana or energy
-Ult ensures few escapes
-Not teammate dependent
Cons
-Has trouble late game vs fast AD ranged champs, like ashe
-Slow auto attack
Items
Doran's Blade
A large part of playing a successful Garen is being aggressive early. Good Garen's can stop opponents from generating a good CS. Doran's helps with this mightily.
The Brutalizer and 
Avarice Blade
These give you damage, gold, crit, and CDR. For a pretty cheap price. You do not want to combined them into Youm's early because you don't have the AD for the haste to be a large benefit and you lose the passive gold gain.
Frozen Mallet
Many people find this choice interesting on Garen since the unique doens't get activated by Judgment. It does provide damage and a lot of health - two very important things for this tank/dps build. Even with the unique not procing off of judgement it is very strong on Garen, since he has trouble with being kitted by ranged champs. FM ensures once he closes the gap it stays closed.
Youmuu's Ghostblade
You wanna complete Youmuu's here (mostly because you finally have the damage to need attack speed, and attack speed is important with your Frozen Mallet). Make sure you remember to use it's activate. DON'T activate it while spinning. Typically you first Decisive Strike then Judgment and follow it up with Youmuu's Ghostblade so you can keep chasing them down and have a bit of attack speed.
Atma's Impaler and 
Warmog's Armor
These together provide a large increase in damage, crit, health, and armor. Once you complete these Garen might as well be a tank, and he still hits like a truck. A lot of people don't like Atma's but with this build it gives you 86 damage, 45 armor, and 18 crit.
Infinity Edge or 
Aegis of the Legion
Ie is purely damage - pick this if feel like you are difficult to kill (which is normally the case with your build thus far). Sometimes at this point if I find garen being too squishy still I get aegis of the legion instead. If someone gets super fed early, I may build aegis before I complete my Youmuu.
Boots
My typical rule for Garen's boots is go with 
Boots of SwiftnessUNLESS there are 3+ AD carries or 3+ AP carries. If that is the case get the respective resistance/tenacity boots.
Wards
If they have a jungler (and they should) you always want to keep wards on the river path to your lane. This lets you play aggressively and out of position at times. With garen sometimes you wanna push them into their tower to hurt their CS. This is mostly made possible by proper ward positions. I typically start to get wards after Brutalizer and Avarice.
Tactics
How to deny CS
One of the best things about garen is how easy he can deny CS against a lot of comps, sure there are some comps you can't (I hate you singed + cait) but if you are able to it's great for your team. This is much easier if you have a ranged lane partner (this is something you should typically shoot for).
This is done fairly easily. Once you have the paths to your lane warded you want to push them into their tower unless you have poked them enough to start thinking about going for a kill. If you are ready to go for a kill let them farm their way back to about the river. Remember, if they are forced to fight on a tower you are hurting their cs typically, which means costing them large amounts of gold.
To do this you bush hump like a pro. If they start farming wait a few seconds then pop decisive strike and run out. Get a good 
Judgment on them and their life will drop rapidly. Make sure your lane partner is aware of what you are doing so he is ready to switch from creeping to killing anytime they try to farm. Some people have trouble doing this because you spend so much time in the bushes. Remember if you stop two people from farming by not farming yourself it's good for your team - you still have a partner getting fed, while the other team is losing 2 people getting fed. Also, once you push them to a tower and can run out and judgment minions, you'll get plenty of creeps.
Once you've pushed them into a tower keep them there (provided wards are up and you're not ready to go for a kill yet). Every time your wave of creeps meets their wave of creeps run out and Judgment the creeps.
How to get first blood
Garen is probably the easiest champ in the game to get first blood with. The most important thing is to let your lane partner know that you are going to be going for first blood, that way he's prepared. Let him know which target you're going try to burst down. If he has 
Exhaust make sure he knows you will both initiate and use yours first. If he has 
Ignite tell him to use it as soon as you go.
Stand in the bushes and wait for them to start farming the first wave of creeps. You want to make sure they are about halfway up the lane. Initiate with 
Flash, followed with an Exhaust so they can't run, then spin and auto attack.
Note on first blood: you don't want to innate this engagement at 50 seconds in. If you fight to early and you almost kill them back they will just teleport back home, heal, and get their in time for first wave. If you are gonna hurt them, hurt them when it will cost them a few waves of creeps.
-More to be added in this section-
Build Guide
Discussion (3)
Greater Mark of Scaling Magic Resist for their marks as Garen. This is viable choice between 1.2 MR at 18 or 1.6 ArP. I personally prefer the MR because MR is a very difficult resistance to get on an AD champ - there are no items that both contain MR and give Garen his other desired stats. And in my experience not being able to take a hit from malz, or a fear/drain from fiddle is much better than doing about 25 more damage a swing.
Greater Quintessence of Health you start with nearly an extra 200 health. This allows for extremely aggressive game play and makes Garen's passive very strong early game.
Archaic Knowledge
is well worth it, since it dramatically affects your ult 
