NEVER use your Decisive Strike (Q) to farm or kill minions while in the laning phase. Any good opponent will know to start attacking once it's been wasted on a minion. Use Decisive Strike to make your foes keep their distance. Whenever they come close to you for cs, pound them with your Q, and if they dont learn to back off, the damage can build up.

When farming with Judgement (E), only use it to farm while you are under your turret. Garen is a great farmer, but he can easily push his lane more than he may want sometimes.

Start using Garen aggressively by level 4-5. Let them get close to you on their own, then W, Q, and E. Once you reach level 6 and upgrade your ult, i recommend only using it to finish off enemies, not to initiate damage.

If you are raping your lane and pushing the enemy turret, you can do some effective poking at the tower. Once the enemy tower focuses a minion, press W, then Q, then rush to the tower, deal damage, then run back behind your minions again. Do this regardless if the enemy champ is camped under the turret or not. He wont be able to do any serious damage with your shield and speed boost up at once.

WARDS, WARDS, WARDS. During the laning phase, make sure you ALWAYS have your river warded. Wards can be the difference between winning and losing a game. Also, make sure your bot lane support has one of the two bushes against the edge of the map warded. This is where your teleport comes to use. Use it on the ward for a super effective gank on bot lane if it is pushing your turret.

Once you have your Warmog's Armor and Black Cleaver, this is where you start harassing the enemy team. If you have your enemy turret's destroyed by now (which you usually will), you can start roaming and pushing the other lanes. Garen will be more tankier than most champs by this time and will cause enemies to flee once you start running towards them. But DONT overestimate yourself and start turret diving enemies with half or more health left. However, if they are below half, you can usually snag a kill under their turret and escape alive.

Late game is where Garen's damage starts to drop, but his tankyness shines. He will easily be able to turret dive with a teamate to kill a squishy enemy and then walk away with a couple scratches. In teamfights, make sure you are the first one in. Shield up, silence the AP carry, then spin to win. If they do enough damage to kill you (which is like never unless your team sucks xD), they will be left disappointed as your Guardian Angel comes into play. By the time you are alive again, they will be fleeing from the rest of your team and you will live to tell the story :p