Garen was the first champ i played. I used to not like him because I did not fully understand the play style he requires. Also what turned me off to him was I saw a lot of people building him as a tank and I was already using Alistar as a tank. When I discovered he could be played a AD damage it was like heaven. With the right lane partner or even solo top I found he was one of the most fun and one of the more dominating recommended characters. Garen is a lot of fun to play and he is even better when he is built and runes/masteried right. He is a very powerful champ in the beginning and he only gets better as the match goes on. He is great for setting up ganks and a real nice champ to help your team get fed and lead to an early victory.

The items I selected seem to offer a well balanced AD Garen. You get high damage and with the defensive items + Courage your durability is very nice.

Farming with Garen is sort of like farming with Singed where try to find a lane that is not being occupied that much and spin around and wipe as many creeps as you can.

Your Decisive Strike does enough damage on its own so with your Judgment leveled first you will be able to dominate your lane simply by spinning. I level courage last because hopefully if your early team fights go smooth you wont be taking a lot of damage seeing how your silence is OP in the start. And for your ult it's the same as any other champ.

PROS

Strong Champ Early on
Decent Farmer
Very Durable with Strong Damage to compliment it
Great Escape Artist with Decisive Strike and Ghost

CONS

Vulnerable to CC
Cooldown Dependent
No Ability to really help other teammates

I chose Ghost because it is a great way to catch up to people or to escape some near death close calls. I take exhaust because Garen doesn't really have a slow until Frozen Mallet. Also Exhaust helps with getting First Blood and other early kills. Try not to blow Exhaust when the enemy is closer to their turret than to yours because it usually leads to them getting away and you possibly eating some turret if you have some real bad tunnel vision. Trust me I have seen some really bad tunnel vision people.

I would not recommend Garen for Ranked especially if you are in the higher Elo mainly because compared to other attack damage champions his utility is close to non-existent. I have seen good Garens in ranked games don't get me wrong but it would really depend on the rest of your teams composition if he would work well.

The masteries I chose are your basic masteries that most attack damage champion would take. I put masteries in the defensive tree to give me some more resistance for some better lane presence early on since you really don't get the tankier items until the end.

Greater Mark of Desolation - I take these marks because they are pretty much the standard for any attack damage character in the game, and I tried some other marks but I found that the armor penetration ones worked the best
Greater Seal of Evasion - You get Armor and Magic Resistance from the Items you get later on and from your Courage spells natural passive. So Stack some dodge with it and your defense is well rounded.
Greater Glyph of Scaling Cooldown Reduction - The shorter his cooldown's the more Pew Pew he can do, specially in mid-game. I know a lot of Garens usually take Magic Resist or Magric Resist Per level but I like to shoot out my abilities as fast as I can.

After you reach level 6 with Demacian Justice and have at least your Boots you can start to wander from your lane and you can set up some real nice ganks. If you have a jungler they can join in and most times if you have good teammates you'll almost always secure a kill on whoever you plan to kill.

These are the champions that I have laned with enough and they are just my favorites that I thought compliment Garen the most
Lux - Offers a Great Snare and Support to help lock down champions for early Kills
Morgana - Similar to Lux has that nice snare and very nice AP damage to go with your attack damage
Wukong - Wukong is a very strong Melee attack damage champ that with his Crushing Blow can significantly reduce a targets armor and just let you and him melt champions. Also he has some nice burst and his ultimate is a real nice crowd control.
Taric - I like taric because he offers a stun, passive armor, and a heal. He is just a nice overall support for Garen to beat down on while under the awesome Dazzle stun that is Taric
My last pick is one I am still deciding on but if you know a good one try it out and let me know how it goes.
Skarner - He has a nice ultimate that in my opinion is nice to help feed any champion. But with Garens Decisive Strike after the ultimate is over, if you silence the target right after it keeps them from using Flash or Ghost to try and get away, giving you and Skarner a lot more time to pick up the kill. For the most part I found when I lane with Skarner we feed each other and carry the team to victory.

I find if you sing when you're spinning it almost always works for you to get a kill. The song I find most helpful is "Live to Win". Of course you can change the lyrics to SPIN TO WIN....TILL YA DIE!
Trust me it works =D