You need know only one thing when using this build: Don't be afraid.

as if I have to add this.

-Don't get hit by minions when farming in order to utilize your passive
-You can cancel your E mid spin to use your newly refreshed Q OR use your ult sooner
-Don't use your E to farm before getting the Cloak of Agility. You may however use your Q if needed.
-Last hit like a pro
-Remember your E does significantly less damage to minions and jungle monsters. Example: When taking dragon, it is never better to spin (unless you're attacking Champions/Champion Pets)
-The bush is your best friend. Q-W-E-R FTW

There's this nice little item in the shop called a Cloak of Agility. It is your main damage dealing item for the early game. The only thing more important than this item early game is your ability to move. After your boots, always get this item. It also enables you to use your E to farm effectively. Your damage with your Auto-attack, Q, and E will raise substantially with this item.

The next important thing for an Attack Garen to have after Movement Speed, and the ability to crit, is the ability to never die. Warmogs goes well with your unstoppable farming. Remember, a healthy Garen is a happy Garen!

Its time to start doing real damage. Oh I know... you've been fed quite well and you're not having any problems (aside from the possibility of your team not being nearly as successful as you are). See how you will never fall off the map in damage. Once you have your Infinity Edge, the consequences will never be the same!

So everyone is running away from you now that they've had a taste of your ridiculous damage and can do half damage to you at most after being dominated. Rushing a Frozen Mallet will fix that problem.

One item left? This is the interchangeable item. Do you need a Thornmail? A Banshee Veil? Guardian Angel? perhaps another Warmogs? Maybe a Force of Nature? The Choice is yours.

Do not get:
-Phantom Dancer
-Trinity Force
-Sunfire Cape
-AP
-Spirit Visage