So I've been looking through all these Garen builds, they are all ****. Waste of time reading these massive paragraphs, so I'm going to make a all in one chapter. First of all Garen excels in initiating and carrying the team. His a top laner who is a strong harasser. Your starting items should be #1 and #2. Starting #1 is just basically if you have a strong harasser at top, like Darius, Teemo. Staring #2 are just normal starting items. I am used to starting #2 because when someone starts to gank you, Just Q out with boots.
Boots of Mobility
Why did i choose this? Because Garen should have alot of movement speed, even if he already has his Q that makes him fast. You SHOULDN'T get these until you rush Frozen Mallet, I will explain is below. Garen + Boots of Mobility = Fast Movement. But why, you ask why not Boots of Speed. Because Boots of mobility has more movement speed than boots of speed. Just jump in your opponent, once they are killed, just walk and you will gain movement speed greater than Boots Of Speed.
Frozen Mallet
Frozen Mallet is really really important to Garen because Frozen Mallet gives him everything he needs.Health, Damage and most importantly slow. You should rush Frozen Mallet before upgrading your boots to Boots of Mobility. Just Q and that should give you some speed, once it hits your opponent, It should slow them and E while they are running back. Once your R is up, finish him off if your confident he'll die, if not just ignite him if it helps.
Black Cleaver
Black Cleaver is very useful to Garen, It is an optional item which depends on your opponent/s. If your stupid mid fed mid, purchase Maw of Mawtirius (However you spell it, just MoM for short). You should look at your opponent's items, It is much much easier to know what to get. For e.g. You opponent has ability power, you get magic resist. If you opponent has armour, you get armour pen.
Infinity Edge
This item is OP for both judgement and decisive strike (esp. decisive strike) If i remember correctly, for 4 seconds, every second he does 100 damage - 200 damage. So thats 200 x 4 = 800 damage in total (Which is maximumly ALOT) and 100 x 4 = 400. 400 - 800 in damage.
Warmogs
Warmogs is very useful for Garen, his just good, just get it.
Randuin's Omen, It gives Garen everything so it's jsut like Frozen Mallet but no damage and some armour. Can replace Thornmail if you want? What ever floats yoru boat.
Garen's Combo
Q - E (This is where you get them low enough) Second time Q - E - R - Ignite ( If your comfortable )

Sorry, i lied :-)
Just a quick guide HOW TO PLAY GAREN P-R-O-P-E-R-L-Y
This is going to be short i PROMISE!
You should know how to play properly with Garen, Sometimes when your opponent gets close, decisive strike and judegment until half way ending the judegment skill, return back. Once your opponent is low, go for the kill. Farm alot, when i mean farm, LAST HIT minions, thsi is the biggest mistaken summoners make, they never last hit. Why? because it gives your extra gold. But don't farm all day! When your team is about to start a fight, you got to help them! If your fed enough, probably around 3/1 or 4-5/1. GO gank other lanes. It would help your teammates alot.