"The most effective way to kill an opponent is to slice through the man next to him." - Garen, the Might of Demacia.
The reason people lose to a Garen player in the current meta is not his kit or his stats. The reason many low-ELO players lose to him is that they deliberately counterpick a "hard counter" champion (such as Kayle or Yorick), despite having little experience or skill with that champion.
First of all, I'd strongly advise you not to pick a champion that you have no idea how to use, even if that champ has a statistically high winrate against Garen. If you choose Darius or Quinn thinking you'll get an easy win against a Garen main, think again. Because his kit is so simple to learn, many players have more time to focus on Garen's metagame, developing strategies to succeed against particularly rough matchups (itemization, positioning, etc.). Many other champions don't afford that luxury, meaning low-ELO players are still learning mechanics and combos for their champ even as Garen is knocking down their towers. Pick a champion you are already comfortable with, or barring that, start practicing with another champion or two that fit your playstyle and have a high winrate against him.
Second, lets go through Garen's weaknesses. He is a low-mobility melee champion; his only gapcloser is a movespeed steroid (and his only disengage); he has no ranged poke; his passive sustain is negated by poke; he has no hard CC and has weak peel; he relies heavily on items and his abilities to remain relevant; his ult execute deals magic damage and scales with opponent's % missing HP; and his kill potential is dramatically reduced while his execute is on CD. Also, despite being a "juggernaut" champion, Garen is not a solo carry champion. If his teammates are behind, it's nearly impossible for Garen to win the game on his own against a competent opponent, even with a significant gold and experience lead.
So in an ideal world, we'd like to counter Garen with a high-mobility ranged poke champion with lots of armor and HP. That's a tall order, but we can probably fit most of those attributes into a particular champion.
Several melee champions who have strong splitpush and dueling capability are the exception to the rule when it comes to melee. Trundle, Tryndamere, and Yorick come to mind. Yorick can lock down Garen in his W and harass with his minions. (Side note, Malzahar can do the same quite well but his low mobility is a big downer.). Trundle steals Garen's tank stats while AP Tryn outsustains Garen pre-11 (still works despite the E nerf). All three can push lane much harder and faster than Garen, setting him behind.
If you want something a bit more durable, Cho'Gath and Tahm Kench are both high-HP, low-mobility melee tanks with strong poke. Cho'Gath in particular has a sweet knockup, an AoE silence to match Garen's, and an execute that deals true damage scaling with his bonus HP. Tahm has some hard CC in his Q, and his W swallow is strong peel that deals % opponent's-max-HP true damage.
Some ADCs also do well against Garen, specifically low-HP, high-mobility ranged poke champions like Quinn, Vayne, Kai'Sa or Ezreal.
Anyway, that's just my thoughts on how to counter my main man Garen. Good luck on the Rift! :)