Changed from 1 point from Offensive cooldown to defensive cool down.
Changed item build, added a extra Doran's blade to start and removed GA for FoN and order of items built.

I've won 6 out of 7 Garen games since making this guide, I find that a GA makes you too little of a target.

This is a build that I use to dominate solo top lanes. With the current meta, top lane is usually a 1v1 with two tanky dps champions. This guide will explain why I have the chosen masteries, runes and items.

With Garen solo top I try to do most of the following things, zone the enemy solo top, maintain a high far and after 15 minutes or so break that top tower.
I play a very aggressive top lane Garen, reason being, his passive, dish out damage and take damage then regen it back leaving the enemy at lower health. As solo top lane is usually a tanky dps such as Irelia, Nasus, Wukong etc, they mainly do physical damage. To counter their damage I start out with increased armour, hence taking flat armour Seals and 2 flat armour quintessences. There is no use being able to take damage if you can't deal sufficient damage. I find that without armour penetration any damage I deal is healed up easily by champions such as Irelia and Nasus. Amour pen. Marks and 1 armour pen. quint gives me enough armour pen. to cut through and do more damage to them then they can to you. Armour, armour pen. and Garen's passive give you output and plenty of sustain for top lane.

I take a defensive 6-24-0 Mastery tree. I pick up the extra 10 damage to towers and the reduction of 10 armour on towers, this helps you push your top tower and then as a team help take down towers with tower armour reduction.
I go defensively taking armour, hp regen, health and reduced damage from minions as you will often have minion aggro in top lane.
I find it useless to have attack speed as garen, you silence, spin and ulti for main damage instead of plain auto attack, therefore I did not waste 4 points on extra attack speed on the offensive branch and therefore missed out on the bonus 10% armour penetration. I added an extra armour pen. quint to help adjust for this.

Marks: Armour Pen. for extra output early and throughout the game. There is no use having little CC and having no output, you won't get targeted and you will become useless.
Seals: Armour helps you in your solo top lane against a most probably tanky dps.
Glyphs: Magic resist is the most helpful, you can go cool down although i find having extra tankiness helps more.
Quintessences: I take 2 in armour to help resist damage top lane and then an armour pen. quint for the loss in 10% armour penetration in masteries.

You have strong defenses from the word go due to masteries and runes so a Doran's blade is my choice, health and damage. I've found without a Doran's blade as first item that I just don't deal enough damage. After my first early recall I take another Doran's blades and start on my boots. I take another Doran's blade as you want enough output to make your enemy think twice before standing in your damaging spin which will lead you to zoning them or pushing to their tower to damage it. After completing boots I start on Sunfire cape which stacking on top of your spin deals a lot of damage. You should now have enough armour and health to be able to dive towers for short periods of time without much concern. Followed up by a Warmogs for extra health you now become a hard target to kill. Banshees veil is a well useful item, as Garen I like to target their AD range carries so with a Banshees you are able to stop what could be a cc spell and make your way forward in initiation. You are already hard to kill but with a GA on top your enemy has very little hope. Although if you find you are tanky enough and need more damage go for your Atma's Impaler. With 4.5k hp, Atma's Impaler will add 137 AD making your total over 260 AD. Guardian Angel can be swapped out for an Infinity Edge if you are after that bit of extra damage end game or a Force of Nature if they are AP heavy.
The build is a high costing build, don't expect to complete it often. But if made you are very tanky and able to drop all squishies!

Be aggressive in that top lane, starting out with your Q will take off a bar of hp early game to your enemy. After you get your E you should Silence your enemy with Q getting your speed buff and then pop your E on instantly and then pull out immediately once your E has finished and hide in the bushes to lose minion aggro and regenerate health. You can stay in that bush to regen. health and it will keep them back as they don't want to face check. No one SHOULD want to face check a Garen, if they do, silence and spin. If you are constantly pushing them to their tower use your Q to hit that tower immediately and then run straight back out. If then enemy then chooses to hit you whilst at their tower they will take minion aggro and lose a bit of health. Buy wards, you will often be pushed or zoning them so expect to be ganked. If you see the enemy jungler then press your Q for the speed buff and run back to safety, the jungler will often have a slow or snare/stun, if its a snare/stun press E to reduce incoming CC, if it is a slow wait till they slow you and then press E to remove all slows. Activating your Q and then E at the right time should save you from EVERY gank.
After laning phase it is up to you to go with the team on the roam and push those separate towers after ganking. Tank the tower if need be, you have sufficient health and armour after your Sunfire cape. In team fights target the squishy damage dealers. If an enemy has a channeling spell such as Malz, Nunu, Kat, Karthus, or Fiddle ulti then make sure you Q them immediately to silence and shut down their ulti immediately.

This is how I play Garen, may not be the best and it may not be the worst but if you have tried it and found it works well then please up-vote, if you have tried it and find there needs to be some tweaking please let me know what you think.
Hope you enjoy Spin2Win :D