Pros / Cons
Pros: Tons of Damage and average Tanking, Carry most of the time
Cons: Not very Team helping wise, most of the times very depended on for fights (Equaling Underfed), HEAVILY dependent on laning with a Tank.
Farming
Farming with 
Garen is a bit complex. At the beginning, you won't get very many Minion kills unless you make sure you Last Hit every time. But once you get your Judgement, it will be much more easier, granted it'll be easier to be Farm stolen from you. When you get your 
Courage, you have to get Last Hits or your Courage will never get farmed, and you'll be left with not much Defense. Don't use your 
Decisive Strike when farming unless you are in DESPERATE NEED of farming (Not including farming Courage, but Level or Gold wise)
Items
Let me explain my Build a bit more in depth. You can consider this a Tank build because I've got 
Warmog's Armor but I consider it a DPS since I stack AD. People don't see the potential Garen has as a DPS. In short, that's why a lot of them fail, because they play Tank Garen. Anyway, stacking Damage was very simple since i've got two Infinity Edges and also "Atmog's" in this Build. This Build is VERY expensive though, so you need to be fed to complete this Build.
Runes
Let me explain my Runes a little bit. These Runes focus HEAVILY on Attack Damage. I get Flat Attack Damage Marks, Flat Attack Damage Seals, Attack Speed Glyphs, and Flat Attack Damage Quintessences. Nothing else is focused because this is not made to be a Tank Build, nor is it for Attack Speed. Now, I'm thinking of changing the Marks to Armor Penetration, in case you go against a Tank.
Skill Sequence
If it isn't obvious what you need to get for your Skills already. You need to get your Decisive Strike first so when you fight in Beginning Game, you'll burst a ton of Health with just one casting. Next is Judgement to help with your Farming and that little extra damage for when you're chasing them to their Turret. Then you get your Courage and farm it until it's fully farmed at 25 Armor and Magic Resist (This should be all you need for Armor and such). And once you get your Ultimate; 
Demacian Justice you start to rack up the Kills (As well as KS your teammates).
When in fights
When the fights start. You've got to be one of the ones to go in first. When you go in, Decisive Strike the person on the Enemy Team that focuses on Skills (
LeBlanc for example). Then Judgement in the middle of your Enemies, don't forget to do your Courage, so you don't die in the middle of your Judgement. Be careful though, if you take too much Damage, you have to run, use your Decisive Strike to help your escape, if they Slow you, use your Judgement to take the Slow off of you. But you shouldn't have to run that much if you get your Warmog's Armor early. Always focus the Squishy, Low Health or not, your Decisive Strike will melt their Health insanely if they don't have enough Health or Armor.
Team Fights
So when you are initiated in a Team Fight, the first thing you need to do is focus the Squishy character, doesn't matter if they're Full Health or Low Health, your Decisive Strike will do a lot of the work for you, if they're about to get away, and you can't use your Decisive Strike to catch them, use your Ult, 9 times out of 10, it will kill them. Now, if there's a Champion on the Enemy Team that you know will take you down with, let's say a Burst ( LeBlanc). When you go in, you need to Silence that Champion with Decisive Strike, then chase the Squishy, hopefully in the time that the Enemy is Silenced, your team will kill that person, and you won't have to worry anymore, in that case keep chasing, but be careful, you will run the possibility of being Ganked by the others on the the Enemy Team in an effort to protect the Squishy. So chase, but be smart about it.
When Pushing
Pushing with Garen is quite destructive. When you get to a Turret, use your Decisive Strike whenever you can so you do a lot of Damage to take it faster. If you're ganked, don't forget to use your Decisive Strike and Judgement to get away and escape. Use your 
Flash if you have to. If you've got a Tank on your team, HAVE HIM NEAR YOU AT ALL TIMES! You will need him to Tank the turrets and Enemy attacks while you dish out the damage back.
In the Enemies' Base
The Enemies' Base, this is were things will actually get easy. If they charge at you on at a time, just take them down one by one, but if they all attack at once, don't be afraid to attack, just Team fight them, remember! Focus the Squishies! Don't retreat unless you ABSOLUTELY have to! Remember, stay with the Tank if you have one.
Summary
I've been using this Build for a few Months now, and it's worked for me quite a bit, so I recommend you try this a few times, you may not get it at first, but it'll come to you at a later time. Partially because the Runes help A LOT with being DPS Garen. Thank you for Reading up on this Build/Guide, if you liked this, please vote for this! If you've got any concerns or ideas for this Build, just leave them in the Comments below, or you can PM me. Thank you
Build Guide
Discussion (1)
Olaf Health/Damage Build/Guide. When I come out with more Builds, they'll be much more different than these two. Thank you for looking, and vote for this! (UPDATE 3/4/12, I've decided to replace two of the 
Infinity Edge for a 
Madred's Bloodrazor for Attack Speed, cause you want to get Damage out faster right? and 
Guardian Angel so you can live more and kill more)
Doran's Blade when you get to your 
The Bloodthirster, sell it on 
Atma's Impaler if you're being underfarmed. Make sure you farm your 
