Garen is by far my favorite champion. With any other champion I would buy multiple mana items just to stay out in battle. With Garen he need no mana so you can focus on health, and damage items. My one problem with Garen was I couldn't find the right build for my play style. Being a Tank was to boring, and being straight dps was a good way to die in every team fight. This build is my way of solving this problem. You could call it Tanky DPS but it is more of a hybrid. During the I will refer to it simply as GG (Good Garen), please enjoy.

Pros

Good damage output.
Good jungling after Bloodthirster.
Easy to get kills in team fights.

Cons

Bad HP
Bad at helping team at times

I hate runes! OK had to get it out. They just suck. You waste IP (or RP if you waste real money) and all you get are small bonuses that really don't make a big impact on the outcome of the game but if you MUST get runes this is my suggestion.

Greater Mark of Desolation x9
Greater Quintessence of Desolation x3
Greater Seal of Critical Chance x9
Greater Glyph of Scaling Cooldown Reduction x9

Once again... real basic. Armor Pen is great for Garen so lots of that. The critical chance is good for farming minions early game, and the cool down reduction... come on it's Garen. (note: there are more expensive cool down reduction runes but not worth it in my opinion.)

NO UTILITY!!! No point at all for Garen, offence and limited defense. Simple right?

Offense

Lots of damage in offense and upgrade your Exhaust spell.

Defense

For defense upgrade armor, magic resist, and dodge chance. This has worked best for me but as always feel free to experiment (on coop vs AI in my opinion)

Early Game

Right away get Doran's Shield. It's nice and works good with Perseverance. Next which most builds get for Garen is Boots of Swiftness. This makes him move faster for ganking. The next item finishes you early game. A Sword of the Occult.

Mid Game

This is odd for Garen but works. Bloodthirster is next giving great life steal and damage so win win, plus Garen can even jungle. At this point alot would want a Phantom Dancer but instead I would get a Black Cleaver. Attack speed and damage plus knocks down more armor so good idea

Late Game

Next item is a Infinity Edge. This will surprise the other team because it does not fit your damage based build adding some crits to the action. This will be a godsend late game. Last item should be a Youmuu's Ghostblade. This should add some needed cool down reduction and the other bonuses we've worked for.

Other Items

Phantom Dancer. This is overkill and looks cool but should only be used if you are getting constantly ks'ed
Warmog's Armor. After you get Bloodthirster if you getting killed a lot you can get this but also consider scratching Youmuu's Ghostblade and getting a Atma's Impaler instead.
Sunfire Aegis. Lots of people like this item for Garen (myself included) but for this build it's just not necessary. If you want it be careful with what item you replace. (under NO circumstance replace Bloodthirster. It is your lifesaver in this build.
Banshee's Veil. Good idea if you are getting killed by slows and/or stuns this is good quick charging defense item, but you will end up compromising a damage item and that could cost you.
Frozen Mallet. Good hp and damage. The slow can help but with Bloodthirster you can just get red buff.
Leviathan. This can replace Sword of the Occult if you would rather have health bonus instead of damage but Sword of the Occult is better in my view point.
Doran's Blade. If you want to replace Doran's Shield for some early life steal but I would advise against it.

What Not to Get

Just in general NO AP!!! Garen is not AP so stay away from it all together, and of course no mana items for obvious reasons.

This is easy. Max out Judgment and get Courage only once early on. Get Demacian Justice whenever possible and get Decisive Strike only when Judgment is not available to level up.

What to Use

Exhaust. Good for chasing or getting away. This is a must in my opinion for Garen
Flash. Good for starting a fight or running away. In general a good idea.
Ghost. Good but not as good as Flash but good idea for some games.

What NOT to Use

These are just the worst ones so there are other bad ones but these are the worst
Clarity. Garen has NO mana so no use EVER.
Revive. Waste of space. No point, at best late game you can get back to action fast but long recharge and not that effective.
Smite. You do enough damage to minions. No need for it.

Please comment, my screen name on LoL is ccollins so add me if you want. Good luck and be a GG.