How to Garen
I wrote a 700+ line program and a 300+ page guide that details the best runes, masteries, and offensive/defensive item build on Garen.
I will summarize it here and show images.
First off, 19 armor pen runes are the strongest all game long. This chart looks at a typical adc with 88.4 (averaged over 20 ranged AD champs) armor from runes and 42 magic resistance. http://imgur.com/a/es364?Noredirect
Although the 19 armor pen runes are not shown, you can see that even the 12 armor pen is stronger than the others.
12/18/0 mastery is also the strongest.
Next, putting points in E in early levels is a waste. It does very little damage increase against a Darius with scaling resistances. http://imgur.com/a/S098s?Noredirect
Go for the Q max. You can judge for yourself if long sword is better than cloth armor start.
I spectated several games on twitch.tv and averaged the W passive increase as such: http://imgur.com/fQv9URu?Noredirect
Thus, with cloth 4 start and no additional defensive items, Garen's effective HP looks as this (Note that pHP means effective physical HP and WmHP means effective magic HP with W active, etc) : http://imgur.com/44iiRBL?Noredirect
As you can see, the scaling HP yellows is better than the scaling armor until level 14. The more defensive items purchased pushes this point to around level 16. Thus, I recommend scaling HP yellows.
A look at 2 defensive items on level 18 Garen: http://imgur.com/saLxySN?Noredirect My choice is Deadmans/Randuins (depending on the enemy) + Spirit Visage.
A look at 3 defensive items on level 18 Garen: http://imgur.com/UwRMC73?Noredirect As you can see, the best defensive items involve both resistances and HP.
When I play Garen, I prefer at least 3 damage items. Anything less feels like a waste. http://imgur.com/a/29ziI?noredirect?Noredirect
However, the best 2 damage items are Black Cleaver and Infinity Edge. The 300 HP and 20% CDR from Cleaver is stronger than the double digit overall increase in damage that Ghost blade provides for Garen over 3 seconds.
My build path is usually this: I start long sword and try to buy tiamat asap. If I back and have enough for BF sword, then I will get that first instead, before completing tiamat. Follow that up with boots and complete Infinity edge. If you are ahead, start the components of Black Cleaver. If not, get a spectre's cowl or chain vest/wardens mail and then complete Black Cleaver. Finish your two defensive items/Cleaver and then upgrade your tiamat into Ravenous Hydra. The chart shows that it is the best upfront AOE damage item, next to IE, on Garen.
So where does Garen need a buff? His base HP or his HP/level. His early game is very weak and these changes will help push him in the right direction.

: In carnage I bloom, like a flower in the dawn. Maybe you have not found the area where you bloom in math.
and crit with Q on a minion (may take a few attempts). If you don't feel like counting, here's the numbers you could possibly get on Garen level 1, with Rank 1 in Q:
