Typically you're conceding early because Garen's early game is awful. Once you hit 6 + get berserker's greaves you start coming online. Level 11 is a huge spike becauseyour ult does much more damage based on how many points you have in it. Level 16 is cool but also doesn't matter much because you're typically overkilling people late game.

Play Garen like a hypercarry. This champion late game with farm is almost impossible to stop in the sidelanes.

Always take yellow trinket and sidelane. Try to dive your enemy laner, get tower damage by ignoring their harras or using your prio to flank an objective teamfight. You want to use your quick rotations, super fast waveclear and infinite sidelane sustain to dominate everyone you meet.

Rank 1: 120 + 20% Max HP
Rank 2: 230.769 + 23% Max HP
Rank 3: 333.333 repeating + 25.9% Max HP

A problem that I see often is that Garen Players tend to play very stock standard, almost like they're in LCS. They'll push waves, take camps, and show up at teamfights. I think this is because Garen is viewed as a learning champion/honest champion, so people don't try and go for plays/picks with him. What I started to realize is that in the mid game, making plays eg: taking sweeper and sitting in a buff bush/shadowing a teammate is really powerful and is underestimated, espcially on a champion with as much burst as Garen.