Hi, I'm LegenDragon, a Diamond euw player. I'm not from an english-speaking country, so please understand that I can make mistakes. Thank you.

Garen, the Might of Demacia, is sadly out of the meta, because it is too easy to kite him.

However, now, Garen can shine again in the Summoner's Rift ! Follow the guide !

Early game :

First, you'll have to lock your Garen, in the midlane. Yes, it looks pretty strange. But it'll be a big surprise for your lane opponent. If you are able to farm with champions like Kassadin, Talon or any melee champ against a ranged champ, then you won't have so much problems.
You have to play safe in the early game. If you play against a melee or an AD champ, usually, you'll have the advantage in the early game and you'll be able to win your midlane, but against AP ranged matchups it'll be a bit different.
Try to farm as much as possible, use your W when the enemy midlaner wants to poke, and wait your jungler. You'll be tanky in the early game, so no fear, there's no risks if you play safe. Don't try to attack the enemy, don't try to trade, focus on farming.
Wait your jungler. When he comes, if you think you can kill, you can flash + Q to instant silence your opponent in order to make the gank easier.

If you have rushed your Spectre's Cowl against an AP, you won't have problem to farm, and you'll even be able to take some trades, sometimes, but always short trades : Q and escape with E through minions. It'll pressure your opponent.

Mid game :

You can start roaming after you have pushed your lane, for example if the enemy midlaner is back. Your roaming will be pretty strong because with flash and ghost, you can quickly get close to the enemy, and take it down. It's pretty easy to gank with Garen, especially if the enemies are pushing.
If they are not, you can still try to dive them : your W gives you passive tankiness, and active tankiness : use it correctly and dive will be easy.
You can take part of all fights on the map, your Garen is really strong with his Black Cleaver. Also, try to invade the enemy jungler with yours, in order to take the vision and steal his buffs. If you see the ennemies, run into them : you are Garen, and your team will follow.
When your protobelt is up, you'll be able to do some awesome catchs, focus enemy carries ; you'll just be able to destroy your enemies.
You can start splitpushing if you want, or keep roaming and pressuring enemy jungler.

The protobelt is the most important item of your build ; it allows you not to be kited. With it, the main Garen's weakness is out ! You have only strenghts, its an easy game for you.

The third core item is Phantom Dancer, because it makes you run faster when you are close to enemies, it gives you tankiness in 1v1, and the attack speed allows you to splitpush so easily. Don't hesitate to use it as much as possible.

About the build, when the 3 core items are done (BC / Protobelt / PD) you can choose situationnal items for your boots and your 2 last spots. It can be tankiness, Youmuu for movespeed, or even Infinity Edge if you want more damage !

Late game :

In late game, you won't be as tanky as a normal Garen, but more mobile, so avoid the big fights where you'll be useless and focus on catchs with your awesome moblity, 1v1, and splitpush. You are NOT the frontline. You can still try to come in the fights like an "assassin" to silence the main enemy caster and try to kill him with E + R, but it's not very easy.

TIPS :

Very important : when you kill an enemy, write "DEMACIAAAAAAAAAA !!!" in the /all chat in order to tilt your opponents : if they tilt, it's even easier to win the game.