So this build is mainly setup to start as a tankish build but VERY good at doing damage and finishing a kill with your ult. Your judgement skill stacked with sunfire is an amazing minion farmer to give plenty of gold.

For runes it is smart to add health so that especially early game your health will be off the charts. Also it is very easy to regain that health with your passives regen. Cooldown reduction is always good on characters that dont use mana because the cooldown is the only thing effecting when you are able to use your next skill. Armor penetration is good to just hit throw the enemies especially with your skills.

The masteries are there to add some super defense to Garen, mainly in the beginning, but they also add some shield for late game as well. I also add a little to some of the atk to make me add a litl more punch.

Doran shield is a great start. Adds health, armor, and more hp regen, which that is all you will really need on Garen. Boots of swiftness is a great item for garen because it lets him run in and out quicker, and when you use Judgement you can be sure to catch up to them and keep your blade at their throat. Sunfire Cape will add to your AoE dmg with your spin which will allow you to farm minions easily to keep steady gold. Force of Nature will normally be used unless they have none or 1 useless mage. This also allows you to have HUGE regen later where even if you are running away with 0 hp, in a few secs you will be ready to turn around and kill someone. Frozen mallet is great for this guy because it hives hp to allow you to be a little more tanky and dmg to add to your spin and dmg output, the slow on your atks will make nobody able to run from you and helps your team catch up to deal the real dmg. Bloodthirster will add dmg to your atks and your spin and extra lifesteal is a plus since your atk is growing as well, do know that your spin is unable to lifesteal. Atmas will give more armor for the tankness and then puts all that hp into dmg for your to be unstopable. if somehow its still goin on dont forget to buy some red pots throughout

Basically your spin will be your main atk vs people and to get away. If they gang up on you just pop your spin and you should have them backin off in no time. Decisive strike will be next because everytime it's off cooldown you can add atks to last hit minions, or just silence an enemy. Courage will add some more defenses but more importantly allows you to say no to a % of enemy dmg. R>E>Q>W

Basically exhaust is great to slow someone while being chased, slow someone so you can hit your spin fully, or just blind 70% to get some dmg off. Ghost will be great with your spin to catch up or get away since it reduces slows. Ghost also helps catch someone with boots of swift to get some last hits with your ult or start usin frozen mallet.

Garen is a great hybrid of dmg and tank skills. Since you do not use mana, you can use all your skills whenever they are off cooldown. Garens passive is amazing throughout the game but with it early game lets you lane as long as you really need to. Late game when you get FoN and your passive healling you, people will start getn pissed cause your hp will always be full. Early game is meant to establish dominence especially if you are laning with a ranged, support, or even a dps who can jump to a target like kat, jax, xin. That way your champs can get the builk of the minion kills and push back the champs dealin dmg. If you get hit hard just stay back for a wave and let your passive do the work.

Mid Game
At midgame you should have your boots and sunfire and will be able to kill a wave a minions easily allowing you to get gold for your FoN. Team fights will be starting around this time and it is important to be present and hit some of the carries with your spin and courage up, also be sure to silence some of the strong mages or carries with skill. Best silenced people can stil atks like fiddles heal/ult, ryze everything, veiger, etc. If you see one of the tanks geting low hp or even a carry tryin to run, be sure to use your ult to drop the hammer down on them. This ult is best used against tanks and some carries like trynd. Dealing nukish dmg on a trynd will make him say "WTF i couldnt ult!!!" trust me it feels good.

Late game
At this time you will have Boots, Sunfire, FoN, and frozen mallet. You should be pretty beast in almost all team fights. Continue with your midgame strategy and if you need to back up let your team know and regain most your hp quick with no dmg dealt to your team and then get right back in there.

I havent rly made many builds on this site so any feedback is useful so please just leave comments and vote. Be sure to actually try this out though because i go extremely positive with this build almost every game. Enjoy!