Hello I´m level 30 garen player and this is my guide.

My profile: http://www.lolking.net/summoner/euw/22483627

I take per level runes because garen is pretty good early game hero and needs some more stats lategame. He doesn't have hard lanes vs ad champions unless they do mixed damage (ie. Darius, Jax after level 6)

You can replace Greater Seal of Scaling Armor with Greater Seal of Armor if vs adc top.
Or
replace Greater Glyph of Scaling Magic Resist with Greater Seal of Magic Resist if vs tough ap top but I personally don´t because I don´t care.

If building Sunfire Aegis.

Frozen Mallet:
For sticking to targets, it's applied on Decisive Strike and it helps getting the full Judgment on enemies.

Sunfire Aegis:
I used to not to build this, sometimes I don't. This item is pretty much only strong in lane and that is it. I would only get this if you're behind or dead even against a tanky toplaner if you're getting outtraded.
This item is pretty bad lategame.

Atma's Impaler:
You benefit from every stat, it gives moderate damage early game, but due to health increase per level and crit, it scales well into lategame. Build this last.

Runic Bulwark:
Get this if your support isn' getting it. Barring that, buy this if you're vs ap laner and you are severely behind. Gives you 24 more magic resistance than Maw of Malmortius though, so if enemy team is ap heavy (ie. evelyn jungle, elise top, lux mid) you can combine Maw of Malmortius with Runic Bulwark and replace Atma's Impaler with it.

Maw of Malmortius:
Your primary magic resist source. Gives you magic resist, gives you damage. The passive shield is really useful too, in lane and in team fights. Gives you 24 less magic resistance than Runic Bulwark though, so if enemy team is ap heavy (ie. evelyn jungle, elise top, lux mid) you can combine Maw of Malmortius with Runic Bulwark and replace Atma's Impaler with it.

Ninja Tabi:

Really good in lane vs autoattacking champions like Jax or Fiora

Mercury's Treads:

Good vs ap laners but also take this if enemy has lots of snares or slows.

Boots of Swiftness:

Good if enemy only has slows and no cc, in which case you can replace Frozen Mallet with Warmog's Armor. Even so I still recommend getting Mercury's Treads or Ninja Tabi for extra resists.

Black Cleaver
Black cleaver has great synergy with garen's Judgment as it applies Black Cleaver stacks.
It is also an important item to rush against tanky toplaners like Shen or Renekton if you got a lead early. If you can't get 1000g and be ahead at least 20 cs or a kill rush sunfire first.

Randuin's Omen

Core item. It is good vs ad carries, which you will be diving, and the active can slow their entire team if used as a surprize initiation. Since the active scales with bonus resists, it has extra synergy with Courage which gives 20% more bonus resists.

Thornmail

Only really good if enemy team is full ad, then you can replace Maw of Malmortius with this. Otherwise you give up alot of health from Randuin's Omen and ap carry can more easily kill you.

Frozen Heart

Garen doesn't use mana and the cdr is not that useful, Randuin's Omen attack speed reduction is better because you will dive the carries and probably get focused. You can get it if their team is full ad AND they have autoattackers top, jungle and bot AND nobody else gets it (ie. jungle Xin Zhao, Top Jax, any adc bot.

Mercurial Scimitar

It is pretty essential against teams with snares and suppresses. It is okay against burst teams but the Maw of Malmortius passive can save your life so it is better against mages with no snares/suppresses. This item is useful vs characters such as Lux, Morgana, Varus.

Trinity Force

You get a sheen proc, some crit and movement speed out of it. Not good unless you are building glasscannon [garen] with Infinity Edge, Youmuu's Ghostblade, Trinity Force, Atma's Impaler, Statikk Shiv but this guide is not about that.

Decisive Strike:

You can use this to harass your opponent without getting hit by their abilities. Don't use Decisive Strike when too far away though, as enemy might prepare their on-hit skill or use cc/slows on you or they run away until your movement speed buff runs out.
Decisive strike can also be used to escape ganks if jungler only has a slow, and a roaming tool.

Judgment:

Judgment is your primary damage source as it has 1.1 total Ad scaling and highest bonus damage.
At level 5 or 4 (with damage item) this skill can clear a minion wave (without siege minion) in one cast.

Courage:

Courage makes your armor/magic resist items 20% more effective essentially. It also provides damage mitigation and cc reduction. This can be useful if you are the only initiator (though you shouldn't because your initiation can be easily stopped by a slow/snare/cc move)

Demacian Justice:

This is your finisher, it is usually more effective to use against carries than vs tanks because tanks usually build magic resist. It does more damage the more health missing so it scales well into lategame.

Get Decisive Strike first because getting Judgment pulls minion aggro and minions do alot of damage early levels.

Then get Judgment

Then get Courage unless you can kill your opponent in one combo

Max ultimate when possible. Then max Judgment first for maximum damage and waveclear, Decisive Strike second for more movement speed. And lastly max Courage

Pros

Durable
Good damage
Noob stomper
Fairly good mobility

Cons

Snares shut him down in small skirmishes
Non-existent cc
Hard time vs peel team

You beamline straight for the carry, but if you are slowed/cc'd and carry gets in range of his tower don't chase and help your team. You don't have to kill the carry if you can zone him from a fight it's just as good.

The only aoe team synergy I know of is Orianna's Command: Shockwave. Orianna shields you, you flash Decisive Strike into their team, Orianna uses Command: Shockwave, you use Judgment tons of damage. Otherwise Garen doesn't fit aoe comps because it's better to hit 1 squishy than 2 tanky guys and you probably won't manage hit more than 2 tanky guys.

Darius:
You got slightly more burst damage, but he will outdamage you with his passive if you engage.
Darius will usually try to harass with Decimate so if he misses a Decimate you can prep Decisive Strike and turn on Judgment. Do this so you won't be Apprehended back and loose the trade. Nothing else about laning realy. In teamfights if darius Apprehends your carry focus him because you will have armor shred from Black Cleaver helping your carry to kill him and slow from Frozen Mallet to keep him away.

Riven:
She will outdamage you 1v1 but you can soak more damage. Before you get rank 5 Judgment try to just Decisive Strike her as much as you can,
because your rank 1 Decisive Strike does more damage than her Valor can shield her and she gets silenced preventing counterattack. After you get rank 5 Judgment push your lane with it so she has to use her Broken Wings for waveclear and keep Decisive Strike her
until her health gets low enough for 1 combo kill. Remember to Flash Decisive Strike instead of walking up to her because otherwise she can stun you under turret and you die.

If riven is maxing her Valor second instead of Ki Burst then you can win a 1v1 vs her.
In this case she is going to either just passively farm or try to bait you engaging on her with her shield on with Valor + Ki Burst combo. In former case just farm because you and your team can just focus Riven and she melts with silence+cc. In the latter case just use Courage to soak up Broken Wings damage while moving back and when her shield is about to run out (duration 2.5s Decisive Strike animation about 0.5s) turn around and Judgment + Decisive Strike combo her.

Renekton:
Quite even lane. Renekton relies primarily on harassing you with his slice + Ruthless Predator + Cull the Meek combo. You can counter this by turning on Judgment and Courage when you see him dashing in and then Decisive Strike him. You should come out ahead in damage. He is trying to sustain with his enraged Cull the Meek, Decisive Strike + Judgment combo him and you will out-trade him without his ultimate because his Cull the Meek is his primary damage source.

If renekton plays passive, you want to constantly Decisive Strike him without getting hit by his Cull the Meek. Easiest way to do it is just walk up to him and use Decisive Strike when you're in melee range (not just running at him with it from behind minion wave). He will try to sustain with enraged Cull the Meek. When he does this, Decisive Strike + Judgment him because he cannot trade you back without his ultimate. If he uses ultimate defensively don't tower dive him even if you have alot of health left. This is because his ultimate can also sustain him through your damage if he bought some tank items and he can also stun you under tower when your silence runs out. If he uses his ultimate offensively; Decisive Strike him and run away until it wears off. Same thing if he is tower diving you.

Renekton can opt to either buy tank items or buy ad caster items, either way this will not influence the playstyle of the renekton but YOUR item choices which are very important in laning:

• If renekton rushes Sunfire Aegis, and then opts to get Randuin's Omen second with Mercury's Treads or Ninja Tabi, get Sunfire Aegis yourself and then get Black Cleaver with boots of your choice.
• If renekton is rushing The Brutalizer into Black Cleaver and Bloodthirster build Ninja Tabi, Giant's Belt, The Brutalizer, Frozen Mallet and then Black Cleaver

Shen:

You can't do anything to him at any level really. He will just spam his Vorpal Blade at you and heal if you try to do anything. Get Avarice Blade, merc treads and lots of potions. Try to keep him pushed to the turret because he got no waveclear skill and sit in a bush to regain some health. Keep river warded at all times.