HEllo, this is my first guide, but dont be fooled as I have played 400+ games with him. First off GAREN IS NOT A FLIPPING TANK. GAREN HAS ONE POOR TANK CC ABILITY WHICH IS SILENCE, PLUS HE HAS HIGH SCALING ON HIS ABILITIES WHICH WILL BE A WASTE ON A TANK. Now that I got that out of the way, Garen is a no skill spin to win one shot champion that just facerolls 24/7 in bushes. Not really, but he does require decision making, good judgement who to focus, and the ability to not ks in 1v5 fights. Garen requires patience which is hard because he can basically oneshot people at level 2+ due to his insane burst. I really mean it, from like 100% to 0% or low. I really mean this. He is the punisher. If you see enemies getting close to brush, go in and hit them with Q then judgement but get out asap to not get damaged. More on that later. More importantly, BUY AS MANY WARDS AS YOU CAN WHEN YOU GO BACK TO BASE. I.E. EXTRA 200G LEFT FROM A BRUTALIZER: BUY 3 WARDS.

ARMOR PEN RED
ARMOR YELLOW/Dodge(prefer a constant armor over dodge, plus are cheaper)
MAGIC RESIST BLUE
ARMOR PEN QUINTS.
nothing else. Attack damage runes instead of armor pen are very viable, but many people do not have those, but armor pen. The reason is because his Q scales 120%ish(not sure, but doesnt matter, he hits as hard as nasus late game) and because HIS JUDGEMENT CAN CRIT!!
JUDGEMENT: YOU MAY THINK THIS IS STUPID "OH IT CANT CRIT, BLAH BLAH BLAH" IT CAN AND IT HURTS ONLY LATEGAME. THIS IS THE FIX TO LATEGAME MANY PEOPLE MISS. Judgement crits with a sword showing above the enemy hit by a judgement crit. This doesnt say crit and makes a special sound. This does not double the base damage, but instead doubles your bonus damage, which is the attack damage you have. I.E. 300 base on judgement and you have 500 attack damage. you will hit for 1300 on a judgement crit (not counting armor, which negates damage) This can oneshot squishies. Thats why AD runes are viable.

MASTERIES (2.)21/9/0 OR (1.)21/0/9
(1.) Taking all the points said above. WHY CRIT DAMAGE? look at the runes section. JUDGEMENT CAN CRIT DOUBLING EXTRA ATTACK DAMAGE(NOT BASE DAMAGE). This adds 10% more AD to your AD when spinning.
Why defense masteries? Because Garen can use a little more armor and dodge helps him move when he has 1000 minions firing at him which can occasionally make him move faster.
(2.) Why the utility masteries? Because Garen has a really high base damage on his judgement(spin) and wants to get the levels up faster, which give an advantage on the low levels. I.E. getting to level 2 while they are level 1. which means killing them for garen only.

WHY NOT DORANS BLADE, I LIKE THE HP!!. Well, I only recommend this to people unfamiliar with Garen as he is an aggresive champion. I build long sword first with an hp pot, which is almost the same in laning. The reason is to build brutalizer right away