Garen is a naturally tanky bruiser that has high kill potential.

He can still tank just as well with only a few tank items.
Building damage and other offensive stats lets Garen be of more use to his team; providing damage, lockdown, pressure, and periodic tanking- than if he were to be a no damage meatshield that can do nothing but tank and ult other tanks.

- Section 1, Starting Items.

Cloth vs AD champions, Longsword for matchups you can easily win, boots for other.
Doran's blade and shield are generally never good unless the enemy lets you attack them for free.
When you back, be the loser that always wins lane through sustain off potions.
This works espacially well on Garen since his passive steroids off this along with him being manaless. This usually prevents you from losing while you're even or ahead and can slow down the opponent if you're losing.

- Section 2, Backing Items.

If you're ahead and know you can keep up the momentum, get BF sword if you have enough gold and build into Infinity Edge.
If you're ahead and you bought longsword prior, build it into vamp or warhammer into Death's Dance.
If you bought a longsword and are fighting an even matchup, build longsword into phage into Black Cleaver.

For boots, you just have to optimize. If the game benefits for you tanking, build Merc Treads/ Ninja Tabi. If the game benefits from Garen's mobility and or Harass, build CDR boots or Swiftness.

- Section 3, Scaling Garen.
Infinity Edge and Death's Dance are core items for scaling Garen into a high damage front liner that can tank periodically and turn around losing fights.

IE allows him to crit off his spin and chunk people with his Q, increasing his overall damage.
Death's Dance is a great complementary item to IE and allows Garen to heal off any physical damage damage off his autos, Q's and even his spin. DD also provides him with nearly every offensive stat Garen needs such as AD, CDR and lifesteal that Garen can use effectively.

Youmu's Ghostblade is personally a lesser built item but it's an effective item on him, aside from the useful stats it provides Garen, using the active speeds him up and lets him take down objectives with the AS buff.

Black Cleaver is a safety net item on this build and a good half way item, while it is not a bad item in general, it's best to grab items that are more important to Garen during the game. However Cleaver lets a behind Garen scale quickly back into the game so that he can either get more damage or tankier.
What can be said for Black Cleaver can mostly be said for Frozen Mallet. However it provides more chances for Garen to stick on people without them being able to run to safety. Mallet is paired well with Garen's Q, making him able to root people just long enough to weave autos with the Q and speed boost until his abilities are back up.

- Section 4, Utility.
Dead Man's Plate and Sunfire Cape lets Garen get tanky without inhibiting his offensive capability,
Dead Man's allowing him to roam easier when paired with or without his Q and do increased damage on his next auto.

Sunfire is a solid bruiser item that provides tank and damage over time to people around you.

The Last Whisper items are somewhat situational, they can be bad buys if you have no use for them other than Armor Penetration as there are better items such as Youmu's Ghostblade which excels better on Garen, but when you're fighting champions that stack armor and hp or champions that like to heal (like Fiora or Trundle), it lets Garen burst them down easier with the damage that he will be getting.

While Spirit Visage doesn't inherently have offensive capabilities, it works well with Garen and Death's Dance. It steroids Garen's regeneration passive, gives 10% CDR to spam abilities, and increases healing done by Death's dance.

Statikk Shiv is usually bought for when you know how much damage Garen can do. (an example being if you're noticing that Garen is killing squishy targets in under 2 seconds without his ult.) It lets him do a steroided auto attack at full stacks and move faster much like Dead Man's Plate, the tradeoff being tank for crit and attackspeed. Attackspeed actually isn't that bad on Garen even if you have low cooldown;
Garen with IE, Death's Dance, Statikk Shiv and maybe one other damaging item during late game can do more damage to targets that are stationary such as monsters and towers quicker with his autos and Q than if he were to use his spin.
- Section 5, Final Build.
Taking into consideration on what this build relies on. Builds should look similar to this one.

Any items that have the following stats are generally not bad on a skilled Garen player if used effectively;
Attack Damage
Crit
Cooldown Reduction
Movespeed
Offhand Tank