There are only two ways to play Garen, I have recently discovered. He can be played as a statue spinning with enormous health to absorb damage(useless if they just stun you and rape your team) Or you can be intelligent and use MY method. My brother has recently taken to it as well. AD Garen, truly is the way to go. I began playing LoL about 3 months ago, and quickly realized, this was my favorite champion!
Garen is possible the most interesting, boring, useful, useless character ever. What I will attempt to do, is increase, lessen, enhance, and remove these aspects, in that order. I have read many guides, played many different ways, and flopped many different times, as well as having even more successes on Garen.
I believe I can say now, without a doubt I know what works and what doesn't, I know when my ult can kill, I know when I can dive a tower, Judgement a target and walk out the other side. I simply wish to share my ideas, with you =)

To start off, I take 9 Greater Marks of Desolation, 9 Greater Seals of Resilience, 9 Greater Glyphs of Warding, and 3 Greater Quintessence of Desolation. Anyone, with half a decent mind, should know why. I shall explain anyways.

Firstly, I take the ArP Marks over Greater Marks of Strength because although Garens base damage is only 61, it doesn't actually matter when you are just going to use Judgement mainly anyways. Trust me, take ArP, your team will hate you by mid-game if you don't.

Secondly, the Defense Seals are an absolute must, if you want you or your lane partner to get first blood and NOT DIE. Go ahead and mess up with ANYTHING ELSE. Then tell me how wrong you were in a comment. =)

Third, we take MR Glyphs, because well, not to do so would just be stupid, you get more than enough CDR from Youmous/The Brutalizer Combo. Nothing else is viable, so save yourself from those annoying characters with AP abilities.

Lastly, the 3 ArP Quintessence are completely needed, Youmous+The Brutalizer+ 25 ArP from Quints, is a total 60 ArP and 55 AD, totaling a technical 115 AD from that easy combo, OP much?

OK. This is where I expect to be trolled, and I can understand why, because I was hesitant to use these items even though they seemed like they should be in his recommended list. Really RIOT, can we fix that? Please?

I will follow with a list of items to purchase, broke down into conditionals, as to what order to purchase for complete domination. The best part is, if needed, you'll be able to multi-class with my build =)

LvL 1-6. Grab a Boots of Speed and 3 Health potions. Try to get Bottom lane if on Blue, or top lane on Purple. Get to YOUR brush, and instruct your partner that YOU will exhaust if they approach the bush, then you hit E and lay waste to them. Upon reaching lvl 6, burn a combo on the next greedy person to chase your partner, once they have hit less than 1/4 health DROP R IN THEIR FACE, then Bluepill, hit Enter and type /all GetGarened. =) Plants a seed to chase you forever.
Purchases Now:[*] Boots of Swiftness
[*] The Brutalizer - If you have been laning WELL and wanna do even better.
[*] Giants Belt - If it has been a close call, or you are getting wrecked.
( Also ask for a Lane Change if possible )
[*] Buy both if you can^^, Spend remainder on Elixir of Fortitude.

PURCHASE NOTHING ELSE UNTIL LVL 11 IF YOU CAN
Lvl 7-11 This is the fun part, where you have suddenly jumped in damage and ArP, or health, or BOTH is even better. If you have been wreccking, chances are mid will go MIA, call your mid down to you, have them camp the bush, watch your enemies reactions to you...If they seem eager to press, chances are Mid is gonna try. Wait til their squishy is in range of your Mid AND Lane partner. Exhaust him, Hit Q and silence him, His partner will rush in, hit E immediately and tap a potion. Hit W, wait for their mid to rush in, He WILL target you, as you've likely Taken 3/4ths your Opponents health and he'll get greedy to save them.

Let your allies finish them, and run to the bush. As soon as your CD has came back on Q, Hit it, run back to the fight and focus their mid hard. 9/10 times, you'll achieve 3 kills, and hopefully 1 or none of you will die. Take the chance, to push the turret, usually ending Early game and forcing Grouping.

Purchases Now: [*] B.F.Sword - If you have continued to do well.
[*] Chain Vest - If they're AD heavy been giving you trouble.
[*] Negatron Cloak - If they're AP heavy have been giving you problems.
[*] Purchase Armor/Magic Resist BEFORE B.F.Sword ONLY if your team hasn't.

LvL 12-18 This should be where they start to surrender if you have Farmed/Ganked well, the first vote should appear in a few...Ooo there it is xD. Seriously though, at this point you should be ripping apart their carrys, and ALLOWING YOUR CARRY TO GET KILLS. If you see a Xin Zhao chasing Caitlyn down the river, run in, hit Q,W, Wait for Q to hit, Hit E, when he is about half health let Cait have him, just tank it. If needed, Drop an Early R at about 1/3rd Health so she can easily kill him. Fed Charlies = Winning. Err, carries that is.

At about lvl 15, Your team should Baron if they haven't surrendered. HOWEVER, don't just rush it, unless you are all fed as ****. Failing Baron, and Getting ganked there, can ruin a perfect game, and bring a team back from a complete wipe-out. Firstly, push a Lane up to the Inhibitor, TAKE the Inhibitor, Wait until they have a good stream to defend against, THEN Ward their side of Baron, and ALL 5 OF YOU Take Baron.

Purchases Now: [*] Finish Infinity Edge.
[*] Finish Atmas Impaler - If you are doing well, and facing mostly Ad/weak Ap
[*]Finish Warmogs - In place of FoN, finish it before Atmas if you prefer, but stacks are easily achieved by Judgement on minion waves.
[*]Finish FoN - If Negatron cloak hasn't been enough MR/you need more HP Regen and/or Movespeed.

Lastly. Win the Game. Be calm, courteous, and don't KS, for the love of God, don't be an annoying childish, raging Garen. You make people like me, hated before a game even starts.

PS Atmas Impaler with 3.1k health, adds 63 Attack damage. So with max Stacks you will actually have 290 Attack Damage. Not 220. =) that is a whopping 229 Bonus damage for LvL 5 Judgement.
220 + (229x2.5) = 792 DPS with E Crits. Over 3 seconds That's a MASSIVE 2.3k Damage AoE.
WHAT THE ****?!?! Just imagine getting Baron =)

Summoner Spells/Why I pick Them

Plain and Simple. Pick Exhaust. Then Pick Flash. Learn how to use Flash to get you right next to a squishy so your Q hits as soon as you land. They don't see it coming half the time.


Masteries
Garen as an AD/Off-tank, does best with 21 in offense, and 9 in Utility. Take 1 point in Good Hands 3 in Perseverance. 4 in Awareness because more XP = Faster Judgement leveling and 1 in Greed. Because it makes up a little of the time you'll spend not last hitting as much.
( I usually make a complete useful build, on 70-130 Creep Smash.)

Not Hard. Take E, I don't care what any body has told you, Garen's E, is the best AoE available at lvl. Period. Probably the best ability in the entire Game. Prioritize E, Then Q.
BUT If you have maxed W's passive and you have a wave of minions close, take W over Q only, and wipe out those minions for more Armor and MR. Otherwise Take R>E>Q>W

Pros
-Easiest First Blood ever.
-Extremely easy to use.
-Durable, Fun DPSer.
-Coolest Taunt in the game.
-Press E, then /dance RIGHT after.
-Awesome Laning presence, Especially in a 1v2 top. (care with this)
Cons
-If you fail, you fail hard.
-People will cry at you for not tanking EVEN if you are doing good.
-Everything is your fault, even if you just killed 3 people alone.