Welcome to my first Guide :) I hope you enjoy it. I'll try to keep everything short, sweet, and to the poi-- DEMACIA!!!

You want to make Garen defensive when it comes to his runes, with the exception of his Marks. For Marks, I use Armor Penetration, which helps specifically with his Q because it's considered an "Enhanced" basic attack. His Seals and Quints are for Armor and his Glyphs are Magic Resistance for the obvious reason that he's a tank so he can't have enough of them xD

His masteries are made for defense and health to make him harder to kill. You have to remember that everyone hates Garen because he's hard to kill and because of his Silence/Spin/Ult combo so make sure he's HARD TO KILL

Garen's entire build makes him even harder to take down. His Boots of Swiftness along with his Decisive Strike make slows almost useless. You're gonna want to turbo Sunfire Aegis because it's your key item. Once you silence someone and start spinning on them, the Sunfire damage starts adding up and helps you finish things off with your Demacian Justice , especially if you've used Ignite . The health regen from Warmog's Armor along with Garen's passive, Perseverance, makes a scary combo for recuperation. Since the slow from Frozen Mallet only works with basic attacks, Garen's Decisive Strike is perfect because, even though it's an ability, it simply upgrades his basic attacks, so the slow works with Garen's Q. Black Cleaver and Infinity Edge are simply for that extra damage that you really don't need since you're a tank but they're helpful to have.

Make sure you start with Decisive Strike, Garen's Q, and not Judgment, the spin, because DS is better for level 1 fights. But after that, max out Judgment while taking 1 point in Garen's Courage at level 4 and upgrading your Demacian Justice every chance you get. After maxing your spin, max your silence, and then your shield. The reason you do this last is because you really only want the passive from Courage. The actual active part of the ability doesn't come in handy until it's at least level 4.

Flash for escapes and chases, this also works well with the speed boost from your Q.
Ignite for those pesky tanky builds that crave some extra damage and for the ones that try to get away. Catch up with your Flash/Q and land an ignite to finish them off.
Exhaust also works well as a replacement for Flash but I prefer Flash.

Farming is made easy with Garen's spin and the Sunfire Aegis. At level 5, without the cape and Judgment being at level 3, you can spin once to take care of the mage minions. As soon as you get the Sunfire Cape, all 6 minions become easy and the bigger one only takes a couple more hits.