alright, so there is a reason why garen is almost never seen in pro-play / high elo game:
Garen has THE most counterpicks of all champions IN THE GAME.
lemme name a few:

first of all his weaknesses:
engage - garen has no true way to hard engage - he relies on his enemies misposistioning and running up to them in order to damage them, either that or burning flash.
burst - garen's damage is in bursts unlike other juggernauts whom all prefer longer fights, garen cannot fight for long as his cooldowns are rather long, even with 40% CDR.
Conqueror - this rune is the bane of his existence - it gives the enemy healing, and provides true damage which ignores his armor/MR also it ignores his W.
reliance on ignite - without ignite garen's early kill potential is non-existant
outscaling - garen damage scales on levels yes, but his build path and his kit makes him far weaker late game than quite allot of the other champs. buying
or your own
will go far to reduce his effective tanking. all of these items are often build later in the game, which in turn softens him up to being blown up again.
rune choice - garen has 3 main keystones: grasp, phase rush and predator. grasp fixes his laning, but falls off. phase rush is the best for general gameplay if you know how to abuse it, predator allows him to fix his engage problems a bit.
build path - garen's build path is extremely predictable, which leads to him being easily countered.
simple gameplay - garen's gameplay is VERY simple, this leaves loads of ways to outplay him / deny him.
weakness to ranged - garen really suffers here, ranged champs can kite him, interrupt his passive in lane and generally scale better.
kiting - garen and other juggernaut's main weakness: garen can only use his Q to try and reach you or has to burn flash / disengage.
there are probably more but i think i made my point.
believe me, once you get some more experience you will look back and laugh at garen, poor guy cant get any rest at all! while he might seem super OP at first, that feeling quickly fades once an ashe just completely kites you to ninth circle.
if garen, at any point of the game gets out-damaged in a 1v1 then he will be completely useless for the rest of the game, this guy not only falls off hard, once he loses his lead it is gone forever.
TLDR: learn the matchup and how to counter our favourite bayblade-meme-demacian. once you do that, every lane or game against him becomes freelo unless you are playing against Riste ( the only guy to OTP garen into challenger)
That's all.