Hi people from MOBAfire, this is my first guide here, and this is a guide focused on a build where you find yourself in a 1v1 scenario on the top lane.
In this guide i will show you a rune+item setup and skill leveling that worked really well on my Garen at top lane.
Starting up, a lot of people complain how Garen's Judgment DPS falls off late game, so i designed this setup to fix late game Garen's issues, since his late game is more like on initiating and securing kills with ultimate.
This guide focus on Decisive Strike

Pros Very good damage for a Offtank/Tank Dominating lane phase Able to initiate Very strong passive Ultimate is a very powerful execution tool Easy and pretty fun character

Cons Can be kited Judgment deals nearly poor damage late game Team may accuse you for ksing with ult Junglers without CC may have a hard time while helping you with a gank

For runes, i took the attack damage per lvl Quints and Marks, those are cheap runes, and they kinda solve the late game loss of potential of Garen. I tried the Armor Penetration runes and flat Attack Damage items setup as well, but i had better results while running ad per lvl runes and Armor Penetration items.
My Seals are definetly the flat armor ones, since most of the solo top champions have AD based skills, and all the minions, monsters and champions autoattacks (But Orianna, Teemo and Cho'Gath) deal physical damage as well.
And for my Glyphs i have chosen the flat Magic Resist ones, because a lot of champions on solo top deal magic damage on skills aswell, and i think after some lvls, Garen's Courage will solve the resistance problems untill you get your defensive items. Of course you can choose for Magic Resist per lvl ones, but my personal choice goes for the flat ones.

For my Summoner Spells i take Flash and Heal, flash is a pretty versatile spell, it can help you to get in range to execute an enemy with your ultimate, and helps you getting out of tricky situations passing through walls, and Heal can help you bait enemies into a 1v1 fight when your jungler is arriving, or turn the tides of a duel when you are about to lose.
Other viable spells are Exhaust since Garen got no CC but his silence, this will help your jungler on ganks and probably get a lot of kills, i recommend pick this only if your jungler lacks of cc.
Teleport is also a good choice, it helps you get back to lane quickly or teleport to a ward for surprise ganks.
Ghost I personally don't like this spell on Garen, since he already have a lot of movement speed from his boots and Decisive Strike, but you can pick it if you fell you need to have more mobility.
I think the other summoner spells aren't really suitable for Garen.

So, this build focus on getting stronger as the game progress, but you can still show some strenght at lvl 1.
Try to buy your items as fast as possible and rush to the offensive brush of top lane, stay there in the corner, lvl decisive strike up.
If your opponent facecheck it, wait him to get into the brush, land one basic attack, followed by a Decisive Strike and one more basic attack, then retreat.
This "Combo" Should hurt him a little, and he gets more aware of your strenght and he will play more defensively, after now focus on last hits to reach lvl 2 fast.
When you reach lvl 2, put a point on Courage and start stacking it's passive, when you max stack it, you get a very good chunk of resistances that will help you survive your opponent harass, and paired with your flat resistance runes, you are a very durable early game champion. By now, you should try to lvl Decisive strike as a main priority and Judgment as a Secondary one.
On your first trip to base, you must buy a Doran's Blade, upgrade your boots to Boots of Swiftness and try for The Brutalizer
Your core early game build is Boots of Swiftness, Doran's Blade and The Brutalizer, when you get those three items, you can duel your opponent easily in most of cases, try to score a kill with an unexpected ultimate in their faces.

On the early levels, to harass, open up with a Decisive Strike and unleash a full Judgment due to the bonus movement speed from Decisive Strike, then retreat, they will try to damage you if they are smart, so use Courage to minimize it, then let your passive recover a lot of health while they can't regain health like you, that's a small victory, and should be done whenever your enemy gets out of position, but don't overextend without Warding the river brush.
When they get low on health but are still on lane, and you are already lvl 6, flash in, decisive strike, spin and ult, you shall take nothing more then 2 turret hits, and a early kill is very nice to help you getting your upgraded boots and first offensive items.
Never fully commit into a fight when your opponent is able to deal constant damage, like Tryndamere autoattacking or Singed poisoning, you must always open with a Decisive strike, spin, and retreat, letting your enemy without any reaction time.

When your jungler pings your overextended top enemy, wait for him to get close, try to initiate with a decisive strike, and use Exhaust if you picked it, then start spinning. Just ulti if you are 100% sure your jungler can't score the kill, cause you must not worry about getting kills, since you aren't the main source of damage of your team, you should be happy with assists.
HOT TIP! Don't waste your ulti if your enemy have higher health then 10%/15%/20% according to ranks, since it deals more damage, the less health your enemy have.

Now, if you succesfully dominated your lane and your jungler had succesful ganks, you should be probably starting to build your tanky items, start off with Giant's Belt for a huge chunk of health and try to build a Last Whisper to finish your offensive item, then you can go for a Sunfire Aegis and ask for your team to tank Dragon sometimes, and if the opportunity shows up, initiate the teamfights, but don't forget to turn Courage on, it lowers the damage dealt to you significantly, you will start getting really fed, killing and assisting. Most of times, try to let some kills for your carry or jungler, they will appreciate it, and a fed carry will be a pain in the *** for them.
After your sunfire cape you can either go for a Guardian Angel or Warmog's Armor, depending on what you need, then sell Doran's Blade, The Brutalizer and the Sunfire Cape to get Force of Nature and Thornmail.
With your build finished you will be able to tank tons of damage while still having a pretty high damage output, and with Courage, you will be able to tank the two nexus turrets like a boss for a quick backdoor win.

This is it, my first guide, i will appreciate constructive criticism, opinions and suggestions to help me improve my guide, and if you want to add me on NA server, my nickname is Silverlord88
Have fun using my guide to dominate the games ^^