Garen has been my main since season 1, so I hope I at least know a thing or two about him.
His gameplay is honestly a little questionable right now. The Juggernaut changes made him a lot easier to play, but didn't really change anything significant about how you play him. If you don't feed early you just end up stronger late game.
His speed was nerfed a bit though, and considering the speed of some of the newer champions Garen often ends up in a situation where half the time anyone he catches simply melt, and the rest of the time he can't catch anyone. It's frustrating on both sides, but this is more of a conversation for another day.
As I said in my first post you will want to go for someone who is either really safe or is a great duelist, at least until you understand how to fight him properly. Karma, Gnar, Kennen, and Jayce are all great picks if you're taking the safety route, and Darius, Rumble, Pantheon, and Poppy all duel with him really well.
The big skill each of these champions have, no matter which list you're going with, is their ability to zone Garen. If you know how to freeze a lane you can easily force him to choose between a very unfavorable fight or a ton of missed CS. He does scale pretty well with levels, but if you scale harder with items he will quickly be unable to fight you by himself.
Just keep in mind most people who main knowledge based champions spend a lot of time learning their bad matchups. I personally can do reasonably well against most of the champions I listed. I won't snowball, but I can survive until later on in the game when I can roam around and fight other people instead. Most people don't practice these matchups though, so you should do just fine.