Hello, and welcome to my new build. I'm not level 30 yet, so I have not actually had the opportunity to completely try this build out, but at my level (currently 24 whilst I am writing this), I find this build to be very useful in both 3v3 and 5v5 games. I have recently been playing this build, and I myself have found that I can get a large amount of kills, help the friendly champions push lanes, and also dole out some nice harassment.

PROS:
+ Early Game is great
+ Can scare the **** outta people earlier on
+ Ultimate is a great tool for finishing people off
+ Has a silence
+ Has a damage reduction shield for 3 seconds
+ Mid Game is still great
+ Has a good amount of Armour and Health to survive

CONS:
- Evens out Late Game, making it harder to help your team out
- Compared to other Champions, his CDs are still rather high

Greater Mark of Desolation - I use this for the nice bit of Armor Penetration that Garen will need.
Greater Seal of Critical Chance - Having Crit. Chance with Garen is amazing, as his damage is very enjoyable late game, and it is beneficial that you get some crits to get that extra chunk of damage.
Greater Glyph of Critical Damage - Because of the Greater Seal of Critical Chance, you'll want to have some Critical Damage increase to go along with it.
Greater Quintessence of Desolation - Large amount of Armor Penetration; backs up the Greater Mark of Desolation.

Exhaust
Ah, such a wonderful Summoner spell we have here. We all know its uses. Use this when a person is trying to run away, but don't use it when they've got too much health. Thinking about your Demacian Justice, you have to be able to calculate when the best time is to use it at your level, and for the amount of health they have, as the extra damage scales off of that. Make sure that you co-ordinate this spell properly as well, not having multiple people casting Exhaust on the same person.

Flash
Flash is a really useful spell; both for diving in and running away. If you're exhausted, or have a slow, use Judgment to remove the effects, and run away. If you can't get away solely with that, then use Flash to ensure your escape. It can also be used to get closer to people: try to activate your Courage and Judgment, then flash into the person while they're running away (near a turret, for example), and then, if they have enough health, use either Demacian Justice or Decisive Strike.

Alternatives

Ignite
Ignite is a wonderful spell, especially later on. It is best used when you've either got a few DoTs (Damage over Time spells) on the enemy champion, or when they've got that last chunk of health, and you've ran out of mana. At a higher level, it is still extremely useful for getting that last bit of damage down.

- - - - -

These items allow you to have both Attack Damage, Health, and a large amount of Armour. I will go on to explain the significance of each item.

Ninja Tabi - Dodge Chance, Movement Speed, and Armour make this the most useful for you.
Youmuu's Ghostblade - Attack Damage for Decisive Strike, Critical Strike Chance for both Decisive Strike and Judgment.
Sunfire Aegis - GAREN NEEDS MOAR SUNS!!! No, but seriously, Sunfire Cape is inseparable from Garen. The Armour, the Magic Damage/second, and the Health are all beneficial to Garen's survival.
Frozen Mallet] - The Health, the Attack Damage, and the Slow will help Garen get that [[Judgment to chop the enemy's health down.
Infinity Edge - Has great Attack Damage and Critical Strike Chance to add to the effectiveness of Judgment.
Randuin's Oment - Great Armour, Health and Health Regeneration/5 seconds, and it's Passive make it a great last item, to help make Garen either indestructible or make him a Juggernaut.

E - W - Q - R (if necessary, otherwise rinse and repeat)

First off, it'll be Judgment to get that damage out there. Then you'll use Courage after you get out of Judgment to give yourself that Damage Reduction for 3 seconds. Use Decisive Strike as soon as you cast Courage so that the enemy will not be able to cast their spells, and by now, there should be enough health to effectively use Demacian Justcie. Make sure you're with your team, and that you communicate how much health you think you need to finish the enemy Champion off with your Ultimate. If you do use Demacian Justice and it doesn't kill the enemy Champion, don't worry, the cooldown isn't too long because it has been reduced by the items. Demacian Justice requires a lot of practice to know when to use it at precisely the right time.

Early Game
Earlier on, Garen is amazing. He'll be able to deal plenty of damage to the enemy Champions, and while he's not in combat, he regenerates his health. Make sure you grab Judgment first, as that's your main DPS skill. Once you get enough for your Ninja Tabi, go back and grab them. Remembering the skill sequence above will help you to effectively use Garen.

Remember to continuously hide in the edge of the bushes towards the enemy Champions, and when they come running in, cast Judgment to shave a handsome chunk of their health off. If you have an ally Champion that can slow, it can pretty well guarantee a kill; depending on the enemy Champion/s you're against. If you've been lucky enough, you should have your Youmuu's Ghostblade by now, and should be preparing for Sunfire Aegis.

Mid Game
Garen is still very effective Mid Game. Keep pushing towers and farming at the same time, so that you can grab Sunfire Aegis and Frozen Mallet. Feel free to turret dive if they have a sufficient amount of health, but don't do it too recklessly.

Late Game
This is where Garen isn't as useful as he is Early and Mid Game. He deals less damage than most champions Late Game do, but he's still a very vital part of the team. Having your Infinity Blade Late Game will seriously boost your damage, making you a Juggernaut. But when you initiate, do not be too reckless; remember, you're not a Tank. Cast Decisive Strike on the most dangerous enemy Champion, before casting [Decisive Strike]] and Judgment. Do not focus their tank first either; focus the squishiest Champion they've got. And remember to aim your Demacian Justice at the RIGHT person! It sucks when you hit someone you didn't want to, say a Tank. If you've got your Randuin's Omen, it's either a Rage Quit, or GG.

All in all, Garen is a good champion, and easy to learn. He does not require much talent, as he's pretty OP. Being a Hybrid helps your survival, and helps to kill plenty of enemy Champions. So long as you're taking charge and also being careful at the same time, you should be able to rack up plenty of kills while helping your allies at the same time.

Scores will come soon enough; have been playing a bunch of champions, but I'll get a summary of five games with Garen.

This build is also not only my own, but I was also helped by blooptron and Durpy Durp Durp, who have much more experience than me.