There are only two ways to play Garen, I have recently discovered. He can be played as a statue spinning with enormous health to absorb damage(useless if they just stun you and rape your team) Or you can be intelligent and use MY method. My brother has recently taken to it as well. AD Garen, truly is the way to go. I began playing LoL about 3 months ago, and quickly realized, this was my favorite champion!
Garen is possible the most interesting, boring, useful, useless character ever. What I will attempt to do, is increase, lessen, enhance, and remove these aspects, in that order. I have read many guides, played many different ways, and flopped many different times, as well as having even more successes on Garen.
I believe I can say now, without a doubt I know what works and what doesn't, I know when my ult can kill, I know when I can dive a tower, Judgement a target and walk out the other side. I simply wish to share my ideas, with you

Judgement is one of the most OP moves in the early game.

Great AOE damage.

Very hard to kill.

Has probably the best finishing move in the game.

Has a 2.5 second Silence in his Q.

Decent escape artist: Q boosts your run, Judgement says suck it to Crowd Control, and has a damage reducing shield.

Garen can almost always blow up a squishy mid carry with three button presses. Q, E and R.

Also, Garen shouts DEMACIA and has a big-*** sword.

Everyone is scared of you early game. If they aren't, they damn well will be after you rob them of 300 gold.

You own the bush. It has your name on it. No one is allowed to stay in your bush. Punish them.