The fact is, while skill is important, some items are better than others. Thus, I've scoured the internet (actually, just LoLking) to find the 6 most winning ranked game items for several champs. This is what I found for Garen, and I've added Runes and Masteries accordingly. Keep in mind that this build is based off of statistics, and while I have tested it, that doesn't make it the perfect build, or even the best. However, Garens who build any one of these items tend to win about 57-65% of ranked games.

To give a bit of a background on my experience with Garen, I estimate my win percentage with him is somewhere between 70-80% in PvP. I've done everything from begging the ranged ADC to switch lanes with me because I couldn't touch Rengar, to tanking my ARAM teams to victory, to being that Garen that NOBODY wants to run into in the jungle. I have no typical build to post, as I will go for tankiness or AD depending on how I feel about my team and the situation in general. Yes, I am that crazy Garen that builds Banshee's Veil. Yes, it has won me games. In short, Garen is probably one of the champs I know the best, and definitely one of my better ones.

As a brief overview, bruiser Garen is known for being great early to mid game, and falling off in damage late game. This build is designed to flow with this stereotype, rather than break it. You will slowly move from moderate AD to insane tankiness over time, making you an effective damage dealer early to mid game, and a nearly undying tank should the game last longer.

Flash/Ignite is pretty standard on most champs, and works for most Garens. Feel free to use something different if you do better with it.

The runes and masteries provided are designed to help you early game, giving you plenty of damage, and just a little tankiness. You'll use the items to eventually make a swing into full tank, but you want early kills to feed your progression. If this isn't what you want, feel free to change it. It's not central to the build.

Work on building Merc Treads, Brutalizer, and Phage in whatever order seems best to you. If you can, use your Q to bully whoever you're up against. Farm with your E and your Q as needed, and if you start to aggro, pop your W. Once you hit level 6, use that ult to punish low health opponents, or even kill them. If ranged cc is having you backed up to turret and losing it even after Merc Treads, and your jungler's no help, consider asking another member of your team to switch lanes. Better to be embarrassed now and make a comeback than to lose the game because you couldn't admit defeat.

It's important to beat your opponent to a pulp and get kills as often as possible. If you can get a kill or make the other guy B, you get free farm and level faster. You do that, and you start to snowball. When the enemy team sees you snowballing, they start to gank. If you notice most of the enemy team missing at this point, ask your jungler for help, and tell everyone else to push like they're giving birth. Even if you're shut down, you've just secured your team an easy tower or two.

Eventually, you'll have your Merc Treads, Brutalizer, and Phage, and be working through the next phase of your build. Test the waters in your lane before you initiate the team fight phase. If you're still managing kills, you don't need to move. If not, start roaming. The carry on the other team will not appreciate you suddenly showing up in their lane, but the rest of your team will. Provided you don't roam too aggressively, you can delay the team fight phase until you're tanky enough to not only initiate, but live through it even after the initial focus. Towards the end of mid game, you should have hopefully started Atma's, or even gotten fed enough to finish it.

By now, you should have noticed you're not hurting quite as much as you were. At this point, most fights are team fights, so it's time for a paradigm shift. You've been making the transition from bruiser to tank over time, but now you're going all the way. It's your job to initiate fights, now. Run in first, with your Q if you're confident you can land it. Any cc short of a stun, and you should E. This ensures that you keep your momentum. Meanwhile, your hopefully competent team is getting into position. Should the game last through Warmog's and Sunfire being built, you won't have to worry about anything alone short of a Teemo, and even then, you can win the exchange if you outplay them. That being said, you probably won't worry too much about a 3 second 1v5, either. With your health, it will take just about every ability the other team has, and then some to get a kill, and if they try that, they lose to your team a few seconds later. If they choose not to use abilities on you, you can still chunk enough damage to be useful, so don't forget to spam skills and use that ult on low health enemies. Also, don't forget that if you have a full build, an assist for a team mate to score a kill is much better than you getting the kill yourself.