Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I've seen Garen as a dps or a tank.
but honestly, I prefer him as a tank.
I think he's great early game, 3 pots and your passive keep you laning.
Make use of your passive if you don't have any pots left. You can gain 1-3 levels by doing so.
First base: You can get Boots of Swiftness and start working on your Sunfire.
Second: Either got the belt, or completed your first Sunfire.
Third: Work on your 2nd Sunfire.
From then, get Atma's Impaler.
If it drags to really late game, Frozen Mallet.