This guide was written for a friend, fairly new to the game and new to garen.
This is merely how i would build him in a standard game (ideally) with the general mindset i have when playing.

Flash - Flash.

This is your MAIN summoner spell, one of the most powerful in-game. It has both offensive and defensive usefulness as with it you can escape or engage, depending on your needs.

Ignite - Ignite
or
Exhaust - Exhaust

In a normal game, its personal preference.
Ignite gives that little extra bit of damage with a healing reduction, making it extremely usefull against a champion such as Soraka or Vladimir. I find that it almost always gets me a kill once i have used alongside my full combo.

Exhaust provides you with a slow, to either catch up to or get away from an enemy player and also works really well when fighting melee types with high burst such as Xin Zhao or Tryndamere

In a draft pick setting it will come down to the enemies team comp.
My general rule is, if 4 or more can be effected fully by Exhausts effects (AD champs) i pick it out over Ignite.

Greater Mark of Desolation Greater Mark of Desolation - x9
These are pretty much a standard for AD champions, negates early armor buildup to increase the damage you deal.

Greater Seal of Resilience - x9
The flat armor is reduced auto-attack/physical damage... Q, W, E - Fast, less damage, spinny, enemy cries.

Greater Glyph of Warding - x9
Flat MR gives a nice early game boost against champions with AP abilities.
I would say that are also a viable option and as most things do, it depends on personal preference.

Greater Quintessence of Desolation Greater Quintessence of Desolation - x3
Combined with the ArmP marks these give a 25 armor penetration boost in early game (not taking into account masteries) which means full damage to cannon minions and less negated damage on enemey champions. A real boost to a garen.

Boots Health Potionx3
This is my standard start, these increase poke and sustainability in lane and (usually) ensures that when you go back your buying atleast 2 dorans blades.
Boots will be upgraded after 3 dorans blades and will be either Ninja Tabi or Mercury's Treads, depending solely on the enemy team.
More CC = Merc
More AD = Tabi

Doran's Blade x 3
I have tried many garen combinations, none as good as getting 3 dorans blades. my standard way of playing garen see's me get these extremyly fast and with the 300hp, 9% lifesteal, 30 AD being achieved at a fairly low price... Too good.

Sunfire Aegis
Although its been nerfed Sunfire cape still provides a nice sustained damage output to minions (synergises well with ) allowing an increased farm. Its also the first step in the 'tankier' garen build i like to run giving 450 health and a tidy 45 armor.

Atma's Impaler
Atma's, at this point in the item build, bumps me to around 210 AD and as my next 2 items are usually health based, Atma's will synergise extremely well.

Frozen Mallet
Sell one Dorans, buy Phage to start you off, this gives a bump up in hp aswell as a slow on hit. Then upgrade as soon as you have the 1950g.
Frozen Mallet is a Garens wet dream, even more so with Atma's... With Q running and a hit landing nobody is escaping your spinny-blades-of-death.

Warmog's Armor
I never used to build Warmogs... its another great synergising item for Atmas but i was always skeptical of whether a loss of 'potenital' damage was worth the increase in health.
It was.
With active the armor and magic resistance you have acrued so far is just too powerfull for most champions to handle and with you hitting 4.5k HP+ towards the very late game you become a real terror.

Youmuu's Ghostblade
In honesty when i started picking this item, it was for the movement speed / attack speed proc... it got to the point where i wanted players to have no chance of fleeing me late game and this was just the 'lol' choice.
However, if you break down the item itself, 30AD / 15% crit / 20 AP / 15% CDR as well as the proc, this item is scary good.
In honesty, i should probably get this earlyier (2 dorans and brutaliser, then uprade at the end) but when you have something that works well, why mess with it.


-- Other Viable Items --
The above is my 'ideal' build for Garen, unfortunately... seldom do you get a perfect situation (especially in ranked or against more experienced opponents)

Thornmail
Great armor and a proc which is deadly to high burst champions such as Xin Zhao, Master Yi, or any of those pesky late game Range AD carries.

Hexdrinker
An item that i believe is pretty undervalued.
very strong against AP champions with a proc that can mean the difference between your kill and survival or death.

Force of Nature
Magic res / Movement speed. Not an item i get often on garen but a high AP stack team can be made 'easier' with this.

Last Whisper
More for tankier opponent team comp, another item i rarely use on garen but if the occasion arrises this is an amazing item to get.

There are other items i have missed of, purposely, Infinity Edge for example.
experience tells me items such as this are good and viable on garen.. but as this is my personal experience i cant reccomend anything i havent tried myself.

*Experiment!

My general skill sequence (as shown above) is aiming for strong early game poke.

is very strong at level one, especially with the armor penetration and increased damage masteries and as the silence also effects summoner spells can mean the difference between an early kill or a getaway (assuming they have flash).

i max this first, although Decisive Strike is a strong single target damage output garens famous "spin to win" button is a real terror for the enemy.
2 points in at level 3 and used following a Decisive Strike hit can drop the majority of champions to or atleast close to death.

1 point at level four assures you 5 Armor / MR early game. not a massive increase by all means but any boost helps. its really power is in its damage reduction. 20% at rank one is an amazing amount and is the real rage mechanic for the enemy team.
*It is possible to get this earlier depending on the enemies aggression and damage output, although as garen i feel so powerful anyway i just roll with the punches.

This is your OMGWTF ability... an enemy <300hp (early game) is pretty much an instagib and is an amazing execution tool for those cocky champions who think they have you.
It is, i believe effected by magic resistance. So i would aim for someone a little less stacked (you would mainly aim for the carries anyway, just dont use it prematurely).

Early Game.
As garen is purely cooldown based, his abilites have no cost requirement. Meaning, you can use these at will to farm 'if' necessary. obviously you will need to make sure that:
A, if you use an ability to kill creeps, make sure its going to kill them (this includes properly timing your Judgement).
B, that if you use a cooldown, your not going to need it in the next few seconds to fend off an attack.

Last hitting is obviously most important, however i also play my Garen fairly aggressively... pick your moments to harrass, if you have time to last hit / harras / last hit do that over running at the enemy all crazy styley (as i see most garens do).
Only forget the last hitting if your confident of a kill (or to burn their flash for you to try again soon).

Your ENTIRE goal for early game is to get as much farm as possible. Your build will be fairly expensive so a steady stream of gold is neccessary.

Mid Game
As you transition to mid game and assuming you had a successful early game (gold wise) Garen starts to become a huge threat. your Judgment will be maxed, it will be regening your health pack from multiple sources (with the 3 dorans) and you will find more and more the enemy will be avoiding you.

End Game
If you follow my 'general' build you will be becoming extremly tanky. you will be engaging team fights with your Q, spinning in the middle of the entire enemy team and take very little damage for the 3 seconds of glory.
I feel, at this point Garen can 1v1 any champion (if played well).
Just be the beast you know your are, DEMACIA!

Throughout the Phases
Harras, Engage, Farm well.
Definetly dont be affraid to run at the enemy at any opportunity because whenever i see a Q'd up garen running at me theres only one thing on mind... "OH SH*T, RUN" =p