This Garen build is designed to not only make you do massive damage, but also give you durability throughout the game. Garen could stay in lane forever with his passive and masteries, and he also does great damage early game with armor pen and damage runes.

When considering Ghostblade, it gives Garen massive overall damage and cooldown reduction. You should only realy use it to either chase and escaping champion, get away from bad situations, or destroy a tower. Garen does not have good tower destroying skills, but he has some of the greatest sustainability in the game.

Atmogs late game combined with Infinity Blade makes Garen a very dangerous source of damage late game. Normally, his abilities would not do much damage against most champions later in the game. Infinity Edge and Atmog's helps him get there.

When starting with Doran's Sheild, you want to dominate the brush so that you can safely regenerate. Trying to get your passive off in the lane is a hassle, and can give the enemy player a chance to push hard. Taking the brush advantage early game will give you significant sustainability.

Garen is very item and bonus dependant when it comes to doing late game damage, and that can get very expensive. The masteries and runes you get should give you enough early game farm and enemy kills to sustain you until you finish your Ghostblade/Warmog's Armor.

Armor Pen runes on Garen are also good because it makes Garen's auto-attacks do much more damage than most would expect, considering how badly he suffers from cooldowns early game. Garen does a very consisten amount of damage throughout the game, and can easily carry his team, despite people telling you to be your "team's tank".

Taking Judgement and maxing it as soon as possible helps you harass from the brush, and it also keeps your enemies away from the minion wave. Just keep it ready(off cooldown) whenever you farm and/or camp the brush. It does tons of damage early game and is also good late game to reduce slows and stuns.

Decisive Strike will make you the team initiator, and it helps initiate ganks from either the river or the edge brush. Just like Judgement, make sure to keep it ready as much as possible. Don't use your abilities on minions unless the enemy is not showing any signs of being aggresive.

Courage is Garen's Secret weapon, and can counter enemies very hard late game. Enemies like Tryndamere, Morgana, and Malzahar can't do anything much to Garen because of it's amazing amount of armor and magic resistance late game.

Demacian Justice, Garen's ultimate, is his super secret finisher. It wrecks tanks in teamfights, and finishes off escaping enemy champions. A tanky champion like Volibear can easily take on a 2v1 and win at almsot any point in the game. Garen will go toe to toe with Volibear and take him down easily. Demacian Justice not only does a crazy amount of magic damage, but it also deals damage per health missing. An escaping chmapion cannot run from garen's flash/ultimate combo. The only advice I can give you when it comes to finishing someone at low health is that it takes experience and precision to hit them at the right time. You never want to preemptivley use your ultimate.