Hey and welcome to my first mobafire build featuring Garen, as you probably can see(if you don't have problems with your eyes or something, but then you can not read this either).

For runes,

I take 22 Greater Marks of Potency for an early game advantage.

I take 3 Greater Seals of Intellect for an extra mana boost.

I take 2 Greater Glyphs of Potency also for an early game advantage.

I take 11 Greater Quintessence of Force for an late game advantage.

Take the masteries I have if you have a brain. They give you everything you need, Ability Power and Mana. It can't be better!!

I actually think you should pick these items, anything else is stupid.
Well, you could change one Archangel's Staff for an Dorans Ring if you are looking after extra survivability, though I think you already have enough. Also, you can get an Augment: Death if you want to.

As you can see, I'm picking Demacian Justice in lvl 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17 and 18 as this is the best skill sequence.
This will give you a high damage output. You will kill everything with 100%hp and up.

I am using Clarity and Clarity, you are in need of loads of mana and then this is your best choice. You should never pick anything else. NEVER!

If you want to play ranked with Garen, follow this build exactly. You have a 102% chance to win!

Pros:
If you follow this build, you will certainly win
High damage
Not very squishy

Cons:
THERE ARE NO CONS! THERE ARE ONLY PROS!!!

As Garen is AP carry, you are not supposed to jungle him, it is an bad idea.

Team work!? That is for *******! Go alone, overextend and then whine at your team when you die. THAT is how a real player plays.

Garens Unique skill is that he can make swords fall down from the sky, take this to your advantage and kill your enemies from the skies.

Farm 24/7. This will make you afford your items faster and do more damage. If an AD carry tells you he is supposed to get all the minion kills cuz you are a tank, whine in all-chat and report him.

Veigar can be a big threat sometimes, therefore you should make a big sword fall down in his head and knock him out.

Morgana and Lux can be really annoying sometimes with their snares, therefore you should make big swords fall down in their heads too.

Jarvan IV only came in fourth place, therefore he is mad and wants to kill you(It can be because of you raped his doughter during the race, but I don't think so). You should make a sword fall down on him to survive yourself.

Watch out for Master Yi's CC's, they are really fu*king annoying.

Garen is a real fu*king OP champ, but only if you play him AD. That is why you are supposed to play him AP, no one will call you a noob then.
If an AD Garen kills you, just start whining and report him after the game.