This is my take on one of my favourite, and one of the best (in my opinion) Champions in the game League of Legends. Garen The Might of Demacia!

This guide focus on building Garen as a tough, but still dangerous offtank/bruiser/fighter.

For runes i go offence. but tank runes are also a good choice for Garen.

This is my rune setup on Garen

9x Greater Mark of Desolation
9x Greater Seal of Attack Damage
9x Greater Glyph of Scaling Cooldown Reduction
3x Greater Quintessence of Attack Damage

Good offensive rune build in my opinion. Maybe flat cooldown runes are better, but cd/lvl are only half the price.

As displayed at the top, i use a 9/21/0 for Garen. Gives great protection and offers all thats needed to be that extra tough and bulky offtank, and still has the most importand in offence to be a dangerous fighter. A more offensive mastery setup may also be very effective, but it leaves you more squishy in early game.

Some summoner spells are good, and some are less good. Here are my opinions on whats good and bad for Garen.

The good ones:
Flash My personal choise for most champions. always good to close a gap or get out of trouble.
Teleport My second choise for Garen. Lets you get fast to where the action is.
Ignite Also a great choise for Garen. Specially if no one else has it. great to finish off those close escapes early game, and a good help against healers late game.
Exhaust A nice slow. Helps keeping enemies from escaping. And helps yourself escape.
Ghost Also a nice choice. personally i think its a bit waste as you got a mini ghost in your Decisive Strike

The acceptable ones:
Promote Helps push towers. might be a decent choice, but i wont recomment it.
Heal A good enough spell if you need that extra healing. but with your passive, i just dont think its worth it.
Cleanse You dont have any tenacity with this build, so it might be a fair spell to use. but againt, its just not worth it in my opinion.

The bad ones:
Clairvoyance let your support take this if its needed.
Clarity Mana? keep your hands off this!
Surge Attack speed and ability power? no way. there are much better choices.
Revive If you think about taking this spell, then dont. First of all its not worth it! not in a milion years! second, you will be flamed by almost every single player out there.
Smite only for jungling, so keep away from this!

Perseverance A great passive. Stay out of combat for a few seconds, and then watch your HP go up, slow, but steady. Realy helps to stay in lane longer.

Decisive StrikeA great movement speed boost, with great damage and a silence.
Perfect to chase, escape and engage. Great at distrubting those anoying mages, or that pesky healer.

Judgment Your main damage ability. also removes slow effects and reduce duration on incomming slows while its active. Does great damage if you can keep your enemies in range. Another reason for some of the items i have chosen to this build. Movement speed is a must!

Courage A great active ability to get out of that tower dive safe. or to help you survive longer in teamfights while spinning around. The passive gives a nice boost to armor and magic resist upon minion kills. therefore its importand to get a point in this early. but only one point is needed to get full benefit from the passive effect.

Demacian Justice The ulutimate execution ability. The more HP your target is missing, the more it hurts! But use it against a full HP enemy, and they will just laugh at you. find that balance point where you can execute them. around 15-20% HP. less on tanks and a bit more on squishy carrys. If they survive a hit from this, it was too soon to use it. Practise, practise and more practise. Soon you can execute like a pro.

Take a point in Decisive Strike first.
at lvl 2 and 3 take Judgment and Courage any order you want.

Max out Judgment first. Should be done at lvl 9.
Next max out Decisive Strike if all goes well, or Courage if you need to be more tanky.
And ofcause take Demacian Justice as often as you can.

I go for quite a bit of defense and health with this build, but also enough offensive items to do some good damage.

Im not a big fan of doran items, but Doran's Shield is a good starting item for Garen. Personaly i go for a Ruby Crystal so i can get Heart of Gold fast. Then Boots Upgrade to Boots of Swiftness and get some basic resistance, depending on what you lane against. go for either Negatron Cloak or Chain Vest
Now we need some HP. Get a Giant's Belt and upgrade to Sunfire Aegis Now its situational. if the enemy team have loads of magic damage, upgrade your Negatron Cloak to Force of Nature otherwise you can get Atma's Impaler first. After these items its time to get that Heart of Gold Upgraded to Randuin's Omen
Finish the build with a Frozen Mallet or if needed some more defencive items like Thornmail (If there is a fed AD carry) or Hexdrinker upgraded to Maw of Malmortius (if fed AP carry)

Here i want to explain why i have chosen the items i have for this build, and why i havent chosen some other items.

Boots:
Boots of Swiftness You need the movement to keep people in the damage area of your Judgment and your Sunfire Aegis, also Garen is a bit slow himself, so every movement bonus is good.

Offence:
Atma's Impaler is perfect Garen. Gives damage based on HP (which is high due to the many HP items and masteries). Crit, that improves damage of Judgment. and Armor. (always good)
Frozen Mallet Lots of HP to stack with Atma's Impaler. Some damage. And a nice slow to prevent escapes.

Defence:
Sunfire Aegis Gives good armor. Great HP (to stack with Atma's Impaler) And the best part, an awesome AOE damage that has brilliant synergy with Judgment This lets you deal out quite a lot of AOE dmg during team fights. Also a great help for farming minions.
Force of Nature Many might ask why this expencive item? why not something cheaper like Quicksilver Sash or more dmg effective like Maw of Malmortius? its easy to answer. Force of Nature is worth it! it gives you some of the best stats for any offtank/bruiser. HP regen, and lots of it. great synergy with your passive Perseverance. Gives more movement. Always a good thing. And also gives a large amount of Magic resistance. any team with a decent setup have some magic damage, so its always a help. and the other stats combined, makes this item very valuable.
Randuin's Omen A good item with many features and good stats. Getting Heart of Gold early game gives a good help in getting gold. When upgraded to Randuin's Omen it gives you Armor. HP regen. HP. A bit cooldown reduction. And a great AOE slow which is very valuable when chasing, and in moving teamfights. Also gives a chance to slow attack speed of attacking enemies. A great item for this Garen build.

Items i dont recommend:
Berserker's Greaves Attackspeed is not that importand for this Garen build.
Ninja Tabi Maybe good against heavy physical teams, but still not worth it. let your tank take these.
Mercury's Treads The tenacity might be good, but you get more than enough magic resist from other items. not worth it.
Aegis of the Legion Might be a good item to support your team. Also good synergy with Atma's Impaler due to HP. but still.. i wont recommend it. Other items are better for this build IMO.
Guardian Angel Pure defensive item. Leave it to the tank. The ressurect ability might save your butt sometimes, but still not worth it.
Warmog's Armor Great HP and HP regen. a very nice item, also with Atma's Impaler but you have to get it early to make it count. And its just too expensive. Again, leave it to the tank.
Last Whisper Good if the enemy stacks armor. Might be a decent 6th item. but still wont recommend it unless they have more than 1 champion with 200+ armor. And then again, you are not a tank killer. leave it for your AD carry.

Items i havent mentioned here are not even optional to this build. so if its not listet here and you still feel like using it, its the wrong guide you are looking at.

Pros:
Great survivability. Lots of HP and resistances.
Awesome passive, giving great sustain.
Good movement, and a mini ghost in ability
Incredible execute ability.
Good sustained damage.
Carrys a huge sword :)

Cons:
Slow moving early game.
Prone to cc (other than slows).
Not alot of burst damage.

Garen is in my opinion one of the greatest champions in the fields of justice. He is strong and versatile. too tanky to nuke down fast, but still too dangerous to ignore in fights. And he is a classic "good guy" knight, for those who like to face the darkness in a shiny armor armed with a huge sword.

Feel free to comment, and please vote on this guide :)