Hello, this is my Garen build. First of all before I get spammed, credits go to Hyukk on his Garen build. I've been using his build for 6 years now and I got to 800 elo. This build is still based on crit. Personally, I think this build is great for both 5v5 and 3v3 because Garen can destroy pretty much anyone in the side lanes quickly with his judgement. As Garen, I'm usually 10+ kills with 2 or less deaths.

Items: I get a long sword and a health pot to start, because it helps with the damage dealt with your judgment. Then turn it into a Black Cleaver. Build Boots of swiftness for more chase or running away capabilities. After that I get Youmuu's Ghost Blade. it gives movement speed, just what Garen needs. Don't forget to activate it by pressing 1. Next, build a phantom dancer. Its super crucial in this build. It gives the crit for garen and it also gives even more movement speed. After, build a frozen mallet. It gives more survivability and it gives damage, but the most important part of the frozen mallet, is the slows after each hit. Garen's judgment hits many times and it is very important that you hit them with all the hits you possibly can for a kill. Then, you go for an infinity edge. This is important because it increases your crit damage and it also gives more crit and damage. Those are the Core items you should have, and usually I don't get past this.

Extra items:
Guardian Angel: If the enemy team focus you every team fight, this should help a lot.
Bork: If you are feeling high and wanna be even higher.
Sword of the Occult: If you are killing them fast, this should help kill faster.
Zz Rot: My personal favorite. So toxic that tilts enemy more if youre ahead.

Runes: As four runes, I like to get armor pen red, and quints, health yellow, and cooldown blue. I get cooldown reduction because I think its important, Garen for a kill want his ultimate up at all times if your judgment doesn't finish him.

Tips: Use your passive, Perseverance, to regen your health quickly if you get damaged.
Use bushes to your advantage, stay in there so the enemies don't push, and if they do you can use judgment to take junks of their hp. Also you won't have to worry about losing hp because you can just regen it eventually while the enemies must potion or level.
Try laning with a CC for a better chance of getting many kills early game. (make sure you duo though because this is not season 2)
Farm as much as you can because once Garen gets his skill Courage, he gets more armor and magic resist.
Your Decisive Strike, most people forget that it boosts your movement speed. So with that and your youmuu + phantom dancer you should be across the map in a flash.