Basically, Garen can destroy most tanks if played responsibly and only going for kills when you can but still harrassing.
-Sunfire Cape makes him a minion shredder and also adds extra dmg during team fight or fights in general
-Spirit Visage, even against one or two APs is good for the self healing which stacks off his passive and general tankyness against things like sunfire and sheen
-Randuins Omen, besides the AOE slow, adds lots of armor and health and slows anyone attacking you
-Boots of Swiftness reduce CC time and Warmogs Armor is 1000 health however its best to take one
-Black Cleaver is where you actually start building dmg but also gives you even more health and by now youre very hard to kill if they arent fed
-Ravenous Hydra is the icing on the cake with attack speed and AOE basic attack that also works with his Q and just the little bit of lifesteal that works with the Spirit Visage

its best not to go into large groups till randuins though you should still keep care. this build is meant to keep you alive and hitting as long as possible so even without dmg you live long enough to kill them or push them away without dying.

and like I said, he can destroy most tanks over time, he can even beat Fiora, though thats mostly early game unless you feed off her, which you should try to do. be wary of ganks since his only escape is your Q, the initiator and be willing to turret hug until you can tank enough to win, thats usually how Garen is. Sticking to the bushes is a good way to run in and get dmg before they can stop you.