This is currently a Work in progress guide. I am simply a person who enjoys playing Garen. This is not for any awards, this is simply a guide people can follow/try out. I hope you enjoy.

Update: 10/07/11
I updated the Runes to be more easy to follow.

Beginning Game :

When you first start out in a game, it is most likely your going to get yourself in a sticky situation. You don't want your teammates calling you out for dying for first blood due to a gank you couldn't run away from. This is why you start out with Boots of speed and 2 health pots. The reason why you don't need 3 health pots is because of Garen's Passive.This will keep you in your lane for the longest period of time making your lane hard to push.

Mid-Early Game: If you're in a lane with a ranged champion, you're at a disadvantage. This makes it hard to get in to melee the minions. Building courage up is the best at this point. That way you can use courage mob up the minions and run back before taking too much damage.
Build boots of swiftness when you can, you'll need them when you need to chase.

Mid-Game: Your team has probably gotten into a team fight by now, that usually ends up with a low level champion running away, or that's you in that scenario, popping Decisive Strike with your Boots of Swiftness can close the gap of an enemy (Hopefully your chasing him) or to make a gap between you and the whole enemy team.
I would build a Phage to give an added health boost to you and to help chase enemies to get cash for your build.

Mid/late game: This is the point where you know the outcome of the game. Either your team is winning, losing, or a draw between the two. To gain an advantage over the enemy team, build a Brutalizer, this will keep you constantly attacking at high volumes of damage regardless of what armor the enemies have.

If everything is going smoothly, build some resistance, it will give you an edge in team fights when you have some defense to your added Courage ability.

Early-Late-Game: It depends what you need, if you need more defense, finish the build for the guardian angel, if you need more speed and attack build the ghost blade, if your finding it hard to chase enemies build a frozen mallet. Personally I would build all the items in the order listed, Nothing should stop you if you have all the items along with the Atma's Impaler. It should give you an extra physical damage of about 600.

Late Game: Do what you can, protect turrets, help in team fights, just remember not to over extend and feed the enemy team, only go in to kill something and NOT DIE. I've seen people go into the center of 5 enemies to get a single kill only to die, that is not worth it.

This is still a WIP and I hope you like it.