This is my build about Garen.
This is a Off-tank Dps build to Garen, will give you high Damage and a bit Attack Speed.

Farming With Fortify:
Fortify have a passive that it give you 9 more Damage to Minons, but it only helps when it is not on Cooldown.

Being Turret Dived:
It helps you when you are getting turret dived with low health, use it and try avoiding the attacks from the enemies. If you not died you maybe get free kills.

Dont Want Fortify:
Fortify can be exchanged with Exhaust, But then you will need to disimprove Reinforce, and put it to Defensive Mastery 2/2, and disimprove Archmage's Savvy and put it to Cripple.

Note:
If you got Fortify you should be careful about the other lanes.

When you got Warmog's Armor, you need to start the teamfight if there is no tank on your team, but remember always take W on when a teamfight starts.

When you are in a Teamfight, you can use W and then Q on one of them, and then E and go around the enemies, and after that just use basic attack.
BUT do always Q on a Mage(AP), cause so they can not use their Ability.
For example, if Katarina are using Ulti try hit her with Q.

You can Harass the enemy with Q + E and go back if you lost to much Health,
and your passive will start, and then wait to you almost have Full Health and do it again.
When you will get Level 6 you can try kill the enemy with ulti but only if he got 1/4 health or lower.

I've seen many players using Garens ulti in the beginning, and it is a huge mistake.
He's Ulti is to kill the enemies when they got low health, you can call it last hit, BUT never try to Kill Steal with it, only if they are close to survive (means that are running away from you and your teammates, and your teammates cannot catch him.

You can Farm with Q, but if there is many Minions just use E.
Still try to last hit with Q or Basic Attack.

When you are doing judgment, you must hear:
http://www.youtube.com/watch?v=jw44JdK3p9k
or
http://www.youtube.com/watch?v=zJv5qLsLYoo
or
http://www.youtube.com/watch?v=1FecqjeUOJQ

When you are running away from the enemies, you must Think Like a Cookie.
You are The Cookie and the enemies want to eat you, you must do anything you know to get away, running in to a bush run another way then they think, and spam Q all the time, it will give you movement speed for 4 seconds.