Pros:
-Extreme damage early game with DS and Judgment!
-A moderate silence which has the longest lvl 1 silence time!
-The best last hitting tool in game!
-A strong defensive spell(courage)
Cons:
-Late game is probably his worst part of the game(the damage from Judgment is greater early,mid game)
-Easily killed if he is stunned+focused early game

Decisive Strike: Great early game harassing spell on mages. Charge in silence and charge out for free damage. Great team fight engaging spell as well. Charge in with it and target the squishy mage:) that mage won't do much more in that team fight. I max this second.
This spell can be your saving angel if your engaged by a mage when your at low health as well.Just silence the mage before he is able to throw any great damaging spell at you and Judgment to probably even get a kill. If you can't kill the mage, simply silence him and run away with extreme speed.

Courage: This spell is a great defensive ability(probably one of the best) be sure to always farm minions for this passive(try to not lane with a carry therefore) and activate this when your ganked or if you're going to dive a turret. I max this last.(The active isn't as necessary as the damage from DS and Judgment)

Judgment: This spell is awesome for taking down squishes in early, mid and late game:) Just use DS, run up to a squishy and use Judgment for insane damage. I max this first. If you're chased by a slower you could Judgment as soon as he slows you and the slow will be removed. Also slows applied to you during Judgment have a 50% reduced duration:) bye bye slower-tards.

Demacian Justice:The best last hitting skill in game. Can be used to secure a kill for your team. Simply target the enemy and watch the big,golden sword destroy them:) But don't use this to kill-steal. It's not worth it. Use it when needed instead. I skill up this whenever possible.

As you can most likely guess, you will want to, if possible at all, initiate with Decisive Strike which will make the enemy incapable of escaping(Other than running, of course) for at least 2,5 seconds. But that isn't all. You should always make sure to time it right, and to not just silence the nearest enemy. The nearest enemy is in almost all cases going to be the enemy tank. Unless the enemy has a bum as a tank with no defenses nor HP, which should hopefully, for the other team, never happen, you don't want to focus the tank.

Garen's high early game damage output allows him to harass his opponents very effectively if he is correctly positioned. For that purpose, you have to control the bushes. Stay concealed all the time, going out to take a last-hit or to harass your enemies.

The basic harass move consists in waiting for an enemy to be close enough to your bush to be able to run on him. Then, use a Decisive Strike + Judgment combo to silence and damage your opponent. Silence him first, so that he cannot use an ability to counter you! Once your move is finished, return where you belong--your bush. Then wait until you are healed and start again!

Be warned:

smart players will try to control bushes to prevent you from harassing;
smart players will avoid bushes if they cannot control them, so you will not be able to harass them as effectively;
ranged champions are harder to harass because they can last-hit and avoid bushes;
solo vs 2 makes harassing much more difficult.

It is easier to harass in the following cases:

1v1 or 2v2 lane;
enemy melee champion;
inexperienced or stupid opponent.

For Garen as for any champion, farming matters. As previously mentioned, though, controlling bushes is essential; therefore, farming should be a part of your control strategy, and you should go out of your bush only to take a last-hit. Do not waste your Judgment to hurt minions early game, you have better to do with that skill (and remember that Judgment's damage is halved against monsters).

As time will pass, your damage output will increase (even as tank Garen). Your farming strategy will consequently change: you will just have to rush into the minions with your deadly Judgment activated, letting your ability (and, if available, your Sunfire Cape) kill minions.

Garen's escaping mechanisms are identical to his chasing mechanisms: run as fast as possible! Choose your escape route smartly, getting into bushes to become untargetable, and change directions unexpectedly. Judgment cancels slows, so it may prove to be useful in such situations!

Tank Garen does not deal any significant damage to off-tanks and tanks in late game. If you are building tank, try to focus on disabling the enemy mages using Decisive Strike and spin into the carries' faces to damage them.

Off-tank Garen's end game damage output will never match any carry's, but it is still superior to tank Garen's. Basically, the game play is the same as tank Garen, except you will be able to damage enemy carries more significantly