Garen is a on of the strongest top laner if played correctly.with Garen you can dish out tons of damage while still begin the unkillable tank. with about 4000-4700 HP and more than 200 armor and MR you can tank the damage from the entire enemy team while your team clean's up the piece of the enemy team. With very very mobile skills Garen can also get out of a 5v1 situation if he is stuck in one.

i prefer armor penetration cause it will allow me to go more aggressive in lane and will deal redicoules damage with your E. i take attack damage quint for lane dominance. if you are laning against a ad champion little bit of magic dmg you should go scaling MR. same thing if you are aginst an AP top laner with little bit of physical damage you should go scalling armor just because you would get alot of armor with your items.

Masteries you should take all the defensive points if you are only foucusing on the late game and you know you're jungler is going top help you alot in your lane if needed. But prefer goingwith showed masteries as it will give me more aggresion in lane. You can go 21/9/0 f you want to itwill give sgnificate damage in lane.

As you can see on of my favorite items with Garen is sun fire cape I like it 'cause it deals magic damage per sec and while your e is activated you will deal more damage from it's UNIQUE passive.and it gives him sustain from HP and armor. As i said if you are laning aginst a magic dealer or a magic tank you should build negatron clock or null magic mantle and finish your sun fire cape ASAP. You can also get frozen mallet instead or sun fire cape if your enemy does both magic and physical damage (shyvana, jax etc)

I like to max my E first 'cause it brings a lot of damage, wave clear and it is very effective in a fight as you can cancle it in the middle of the cast and you can ult wihle it's on but it will be deactivated. i max Q seconde because i can escape from almost any situation and i can silence and run away to live. i usually take w las just because the late team fight the w is very effective and it lastes 5 secondes. And when you let your enemy farm under his tower when the tower are hitting the ranged minions you can also harras him with your E. when you are playing against heavy CC champs like Thresh or Leona etc. remmember to use your w just before their stun in that case you will be reducing the stun effect for 45% less.

On spells you should get flash and ignite. I take flash cause it is very important to flash ult someone or flash to survive. I take ignite cause Garen dosen't have to much kill potontial when his ult is down. if your opponent dosent back and he is low you can kill him but with ignite you can surprise him too.

Pros
strong sustain
great early game damage
one of the best tanks in this game
gets 20% extra armor and MR from his w passive.
Cons
Not so good kill potential when your ult and ignite is down
No good escape tool if stuned or snared.
dosen't have a lot of cc

As farming goes try to last hit as much minions with your basic attacks if you have trouble farming with auto attacks use your E to farm and push waves. To have a lot of harass in lane try to Q your opponent after you last hit a minion or when he is standing way to close to you.

In conclusion if you play Garen corectly and you know when and when not to engage and retreat your team will have much easier time wining the gamr