Following the upcoming Garen buff, new updates are bound to be needed, and will be attended to when the buff arrives.

Yes, I'm aware that Garen is usually played as an off tank dps, but this isn't one of those guides, because I feel that Garen, although viable as an off tanky dps, is better played as a tank. Before playing Garen as a tank, some clarifications must be made:
You will be blamed for killstealing if you use your ult well in team fights (just remember, it's really killsecuring). You will not be indestructible. Sometimes it is necessary for Garen to die in a battle, just because those last couple hundred points of health can make the difference between a crushing defeat and acing the enemy team. Even if you are playing against a team that is overly heavy in one area of damage, and you are, say, stacking thornmails, the amount of health you have without Warmog's is very limited, and as such, so are your extended tanking capabilities.
The item builds that I have are very loose, as tanking different teams requires different item orders/builds, but I was able to sum up the basic builds for three scenarios of tanking:
Tanking against an ad-heavy team, tanking against an ap-heavy team, and tanking against a decently balanced team. When I say ad or ap-heavy, I dont mean 3 out of the 5 opposing players are ap or ad, I mean around 4, and then maybe with a Tryndamere or a heavy ap nuker. In short, don't rush to the alternative guides just because the opposing team is slightly unbalanced; simply keep to the base build, and if any of their carries gets overly fed, consider building a little extra resistance to their damage type.

Pros:

Decently high dps early game if played correctly
Courage is a natural fit for a tank, providing a decent item's worth of armor/magic resist, as well as a handy shield.
Presents enough of a threat that gaining aggro isn't too much of a problem
Easy initiation with Decisive Strike, gaining the upper hand over the enemy with the lack of their spells
Naturally quite tanky
Easy to punish melee dps

Cons:

Can be hard to draw fire from an experienced enemy team
Lacking in crowd control
Can be very tempting to leave a fight in order to pursue a low health enemy due to the move speed bonus from Decisive Strike.

Marks:
Greater Mark of Desolation for the Armor Pen, because Garen does most of his damage as physical, except for his ultimate, Demacian Justice, which deals magic damage.
Seals:
Greater Seal of Evasion because they help the most not only early game against those pesky minion hits, but also because they can completely ignore one of Tryndamere's often 500+ critical hits.
Glyphs:
Greater Glyph of Scaling Cooldown Reduction to help with Garen's abilities. Being a mana-less champion, Garen's only limitations on the use of his abilities are his cooldowns, hence cooldown-reducing glyphs. Greater Glyph of Scaling Cooldown Reduction and Greater Glyph of Cooldown Reduction can be mixed and matched, with more Greater Glyph of Scaling Cooldown Reduction the better you want your late game to be, and more Greater Glyph of Cooldown Reduction the better you want your early game to be.
Quintessences:
Greater Quintessence of Health for early game survivability, and the added bonus of more crucial early game health regen/second from your passive, Perseverance.

Doran's Shield:
A great starting item for Garen, as it gives a fairly good starting armor bonus, a good health bonus, making your passive heal you for more, and as if that wasn't enough, more health regen. This is the item that allows you to make using Teleport as a summoner spell viable on a decently fast champion like Garen. This is because this item allows you to stay in your lane because of its added health regen bonus to your passive as well as its own innate health regen bonus. Staying in your lane for long periods of time is very crucial for early game Garen. As an example, your first use of Teleport (providing you don't die) should be once you reach 1000 gold against an ad-heavy team, or 1400 gold against any other type of team. You should at this point recall back to your base, grab your team-dependent boots and 3 Health Potion, and if you feel the need, a ward or two. This is where Teleport comes in handy: once you've made your first recall, you can immediately teleport back to the action, insuring a minimal loss of crucial early game experience.
Aegis of the Legion:
A great item, one that I find very useful in many situations, as it gives nearly identical magic resist and armor bonuses, a handy damage and health bonus, as well as putting the damage and some of the armor and magic resist into a handy aura!
Mercury's Treads:
A good item for a Garen going against a balanced or caster-heavy team as Garen's, giving some much-needed early magic resist as well as tenacity. I only use this item against balanced or caster heavy teams because without the extra magic resist, Garen's armor bonus tends to stay a good deal ahead of his magic resist, allowing him to be easily nuked down early game.
Ninja Tabi:
Used instead of Mercury's Treads against an ad heavy team, as the early game armor and dodge chance greatly increases Garen's already formidable survivability as well as fitting with his dodge seals, taking out a sizable chunk of those pesky auto attackers' damage early and late game.
Thornmail:
A great item, can be exchanged with Randiun's Omen if need be, but lacks the spread out bonuses of Randiun's. With an awesome armor bonus and damage reflection, however, paired with a cheap price, it can be used in conjunction with Warmog's Armor to dish out damage to those pesky Tryndameres and Ashes, allowing you to actually win 1v1 fights against fed ad carries. As of the first edit, you may notice that Thornmail is no longer included in the core or side builds, but keep in mind that Randuin's Omen can easily be substituted for Thornmail if you are confident that the concentrated armor and damage reflect will benefit you more than the more spread out bonuses of Randiun's.
Force of Nature:
Simply put, and absolutely amazing item for Garen, giving him everything that he needs to survive: movespeed, health regen, and magic resist. The magic resist equivalent of thornmail, in that it goes extremely well with Warmog's Armor and Garen's passive.
Warmog's Armor:
This is your ultimate item. No tank Garen should-nay could go without it. The massive health bonus allows you to take in massive amounts of damage, and coupled with your innate magic resist and armor from Courage, keeps you alive for what seems like hours in team fights. Works extremely well with Garen's passive, Perseverance as well as with Thornmail's damage reflection, making you a walking deathtrap for ad carries, and even Force of Nature's passive, allowing you to survive multiple team fights without having to recall.
Randuin's Omen:
A great item for Garen, giving passive slow chance, health, health regen, and armor in one sturdy package. I recommend building Warden's Mail first, then grabbing a magic resist item that builds into either FoN or Spirit Visage before completing it.
Guardian Angel:
One of the only two items that gives both magic resist and armor bonuses, gives you a handy armor bonus as well as a decent magic resist, and a spare life to throw around if need be.
Spirit Visage:
A low-cost item with decent magic resist, and a passive that works extremely well with Perseverance and the passives on your other items, such as Force of Nature.

Teleport:
Obviously a little-used spell, but one that I feel can greatly add to the way Garen is played, more so than Exhaust or Ghost for example. One of the common misconceptions about Teleport is that its only use is as a movement spell. This, however, is not true. Aside from the obvious ease of movement across the map, Teleport also grants invulnerability to the minion it is cast on for the channeling duration. As one can imagine, this has very many uses, ranging from the mundane of being able to keep alive a minion that is being targeted by other minions or champions, to the ultimate use of making the minion immune to even tower damage, allowing your laning partner to be able to attack said tower without fear of running out of minions.
Flash:
A bit more common than Teleport, but just as useful, if not more. I really shouldn't have to explain how to use this spell, but here goes: Walls. You can jump over them. That's pretty much it, although use with Garen's ult deserve some special mention, because it can often be beneficial early game to catch a low-health opponent off guard by tower diving, popping your ult off for the kill, and flashing out over a wall (recommended to have Courage active while tower diving). Also lots of uses for escape, as Garen is very vulnerable to crowd control spamming once he has used Decisive Strike and Judgment. Again, walls mean nothing a Garen with flash.

Although many may dispute the viability of rushing Courage instead of, say, Judgment, one thing to keep in mind before beginning to flame is that this build if for tanking purposes only, and as Garen goes, the only other skill remotely useful for tanking would be Decisive Strike, and as such is given second priority. The reason for giving Courage leveling priority over Judgement and Decisive Strike is a simple one: your early game Garen must be focused on making your late game viable. If, for instance, one was to give Judgement leveling priority over Courage, one might even get more kills in the early game. However, the main weakness of Courage is that you must kill minions in order to level up its passive armor/magic resist buff, and the ideal time for that is in the early laning phase, when you'll be leveling up quickly, negating the possible waiting time between maxing out each level's max buff at higher levels. Another reason is that once the early game laning phase is over, you will have a greatly reduced amount of opportunities to get to max bonuses, as you will often be with melee dps or ap nukers who are determined to push the lane, leaving you, the relatively offensively weak Garen, with very little chance of successfully getting the minion kills you need in order to max out Courage.
The other issue that may be taken by others at my choice of skill sequence is the decision of giving Decisive Strike leveling priority over Judgement. When it comes down to it, Judgement is undeniably great for pushing minion waves, however that is not something you will be focusing on very much in your early game. Additionally, Judgement relies entirely upon luck to get the last hit and thereby increase your Courage buff. Decisive Strike, on the other hand, if absolutely perfect for last hitting, as it can (and should) be used in the following manner on minions: after your first normal hit, quickly cast Decisive Strike on the minion, and after doing so, right click it again to start the auto attack pattern. The beauty of this is that you not only get 3 attacks in in the space of 2, but is that Decisive Strike will likely kill the minion before you land that third hit. Additionally, most of what prevents Garen from dominating the last hitting game is his attack speed, for as a tank you will not be getting very many (if any) items that boost it, and as a result of such, last hitting can be an unsavory affair. This is partially negated by being able to artificially boost your attack speed and damage by using Decisive Strike, allowing you to get an easy last hit. Another use of Decisive Strike is harassing, and while this may sound silly, it is actually a fairly viable tactic. Early game, when you and your enemies have low max health, you will find that Decisive Strike will take down a sizable chunk of said health. Some clarification must be made about harassing with Decisive Strike, however: it works best on melee dps champions, and slightly less well on ap nukers, and not very well at all on ranged dps. The reason for it is this: What you will do is simply attack an enemy with Decisive Strike's handy move speed bonus, dealing damage and silencing them. Depending on what type of champion you just harassed, you have a few options:
1. If it is a melee dps champion, you can either continue to attack and further injure the enemy, or if you feel content at harassing, pop Judgement and spin out, deterring retaliation.
2. If it is an ap nuker champion, you can either continue to attack (for less time than with a melee dps champion, the reason for which will be explained shortly) for a short period of time, (short as in 1-2 auto attacks) or retreat. If you choose to continue attacking, the best course of action is to pop off a couple of auto attacks, then retreat before the silence wears off and the enemy champion gets you back for harassing them.
3. If it is a ranged dps champion... WHY ARE YOU HERE, I TOLD YOU NOT TO DO THIS! The thing about trying to harass ranged dps champions as a melee champion is that although you may be able to do some damage, if they are halfway decent at playing a ranged dps, they will punish you long and hard for it, making you pay (with interest) for the damage you inflicted on them. Furthermore, you can't simply deter them by popping Judgement and spinning out, because they won't care: they can attack you from outside Judgement's range. All-in-all, harassing a ranged dps with Decisive Strike is something that should not be done, unless you are absolutely sure you can get away with minimal damage inflicted.

Is there anything you'd like me to explain or anything of that sort? If so, please comment (inb4 flaming trolls raining from the sky) and I'll be more than happy to help.

Not very many people to start with, but mainly Coo1Pers0n, who kept with me while I experimented with different builds, and was always patient, no matter how horribly I screwed things up.