One thing I have to say is: Never use Garen for tanking unless your team can focus squishies and carries down first and fast. If your team doesn't have enough coordination to do that, they will most likely not succeed. Garen can only hold down CC and the enemies focus for about 2 seconds, but thanks to your Atma's Impaler, Decisive Strike, and your Frozen Mallet, you have enough damage and CC to hold focus for about 3-4.5 seconds. Your team needs to coordinate their fire at whoever needs to be killed first. That is enough time to eliminate a threat and start killing another person. Make sure you don't have a teammate that just throws their spells at the nearest living thing. I can't stress this enough: Warn your teammates about who to focus.

Now, this doesn't mean you have to run a premade every game, it just means that you have to step up and call out who to focus, and you have to place the wards. Some people may call this an unnecessary complication. I call this common sense. Garen is by no means the easiest person to tank with, but once you get it down, he can come up to a level that is comparable to say, Malphite. You just can't initiate as good as him. Many people don't agree with tanking with Garen, but I recommend all Garen players to try it.

I highly recommend that players new to Garen try out my Tanky DPS build first, and step up to tanking later. Luckily I covered this. The build above is my Tanky DPS build, as most players that try out a champion for the first time decide to just take a look at the top items and not read the guide until later. Covering all angles of a champion is hard, but you can find most of the information that you will need by reading my whole guide. It is a bit of a lengthy guide, but I assure you, it is possible to have great fun with it.

Now, let's cover why he can tank:

He is naturally bulky, and when built right, takes a lot of damage. Now, being bulky isn't enough to make him a tank. He needs CC and initiating capabilities. Let's look at that:

CC

Decisive Strike provides a silence which can save a teammate from being annihilated by an AP nuker such as Annie or Ryze.

Frozen Mallet is core in the balanced build. It provides a slow which can stop an AD carry from getting to your squishy member before your teammates can kill him.

Initiating

Decisive Strike gives a nice speed boost that is a nice help when initiating.

Boots of Swiftness make your base movespeed reliable enough to get you into a fight quickly.

Ghost gives a very big boost to your speed. With the mastery to improve it, it makes you very good at initiating.

Making Enemies Focus You

Atma's Impaler has amazing synergy with Warmog's Armor, and gives a nice damage boost.

Frozen Mallet gives you a slow and some more damage for your Judgment.

Sunfire Cape is a core of any build because it has absolutely perfect synergy with Judgment.

This bonus AD will give some nice damage to your Judgment, and when combined with Sunfire Cape, it will make your damage output quite notable. The enemies will surely realize this, and are likely to use their CC on you when you go in. And even if they don't you can CC them, and that helps to save your team from a nice bit of damage.

Pros

+ Great Tank
+ A great mix of health, Armor, and MR
+ Hard to catch
+ No mana

Cons

- Not as much damage as a DPS build
- Not so great at initiating
- None of your abilities are that good at protecting people(No taunt, Snare etc.)
- Cooldowns are annoying

Defensive Tree

Tier 1

Resistance 3/3: Magic resist, nuff' said
Hardiness 3/3: Armor, nuff' said

Tier 2

Evasion 4/4: Goes great with those dodge seals.

Tier 3

Defensive Mastery 2/2: We don't need ardor, so we need these two points to unlock tier 6. Still helps though.
Nimbleness 1/1: With all this dodge chance, this is nice to have.
Harden Skin 3/3: Blocks some damage, not much, but it helps.

Tier 4

Veteran's Scars 4/4: Some nice health to supplement our early game.

Tier 6

Tenacity 1/1: 4% reduced damage will help a lot when tanking.

Offensive Tree

Tier 1

Deadliness 3/3: The crit chance is minimal, but better than none.
Cripple 1/1: An improvement to exhaust is always welcome.

Tier 2

Sorcery 4/4: You're not DPS, so alacrity isn't good for us. Sorcery helps with your cooldowns.

Exhaust: Keeps them from escaping, provides you with the CC you need to tank, and keeps people in your judgement. Highly recommended.

Ghost: Helps you escape, and helps you get to a fight to do your job. Recommended, but replaceable.

Flash: Great for initiating, chasing, tower diving, and much more. A good choice overall.

Heal: Helps on every tank, because you can stay in the fight longer to take the hits. Not recommended by me, but it is just preference.

Fortify: Keeps your turrets safe, prevents tower dives, and helps you last hit when it's off CD. I only recommend this on supports, but it's still good on tanks.

Cleanse: Kind of unnecessary because judgement is kind of a built in cleanse, but if they have so much CC that you need it, then take it. Not recommended, but plausible.

Teleport: Gets you to lanes faster when you recall so you don't lose as much gold or EXP, and good to get to a fight quickly. Not recommended, but plausible.
The Bad

Revive: I've seen tanks use this so if they die they go right back into battle, but the cooldown kills it's usefulness, and you're hard to kill anyway. Plus, you should know when to retreat.

Ignite: Good on burst assassins like Katarina, but since you are tanking, damage is the least of your concerns.

Clarity: No mana = No Clarity.

Clairvoyance: As much as I love it, it's more of a support spell in my eyes.

Rally: I made a thread on the forums about this. Turns out, there is almost no way to make this a viable spell because of the cooldown and the little benefits it offers.

Smite: Last time I checked, you weren't jungling.