This is a build I have used for a while. It gives you enough toughness to survive long enough to escape fights and ambushes, while making you deal out horrifying damage. As soon as you build black cleaver, you can expect to begin racking up kills.

Now here you'll expect me to tell you why I chose these runes. Basically, I bought them and own them and would like to use my IP on champs.

Focus defensive masteries to help with crowd control and such.

My item build focuses on early game farm and late game dominance. I will have an individual chapter on each of the important items.

There are two reasons not to buy potions. First, Garen has good enough sustain with his passive that if you are only harassing when your courage is up, you will be able to stay at high health while farming.
Secondly, your goal is to recall whenever teleport is up. First you use it right away to finish avarice blade, secondly you use it to get free kills with homeguard boots.

Avarice blade is a useful item for Garen in that he can dominate the lane and pick up every minion kill for the first half of the game fairly easily. This will allow you to make good use of its effects.
Also, it builds into Atma's Impaler which works well with Infinity Edge. All of Garen's moves can crit except for his ult, so having this early and lategame crit bonus will increase damage output.

Because Garen has an executing ult, he can kill people extremely quickly. However, he has no crowd control so they can easily get away. The goal of homeguard boots is to give yourself that extra speed which will make you inescapable for a fight and let you take out vulnerable opponents instantly.
These work best in conjunction with the teleport summoner spell.

This is where most of Banox's damage will come from. Its effect works with all of Garen's abilities except his ult, so Judgement will shred armor on everyone it hits.

By the time you build this item, you're opponents are running terrified from you. The only thing you need is that extra slow effect which will make you truly inescapable. Buying this will make you a nightmare to run into alone because there will be no escape.

You've built a lot of health by now, but no armor. Unless you're dominating hard, you need to get the armor right about now because you will start to engage their adc a lot more at this stage of the game.

This will be rather late in the game. Your enemies will begin to take too long to kill, so you need more damage. The easiest solution is to overdouble your damage output with this unstoppable weapon.

You kinda need to get it just because the prerequisites are taken care of. Your avarice blade should have paid for itself by now anyway.

Decisive strike is the most useful for harass, minion kills, and escaping so it should be your first pick. Next, other than taking one in Courage at level 3, Judgement should be your top priority. It is used to rapidly push the lane and for your real damage output.

Pros:
This build focuses on preparing for a strong lategame. You will be doing lots of damage and will be able to survive a long chase from the entire enemy team. You will also have freedom to move wherever you want and start a team fight from any position.
Cons:
The early game is tough because of the emphasis on late game rewards. You don't have any safety nets, so you have to be able to dodge ganks and play it defensively when you need to.

Farming is of the utmost importance. If you can't steal the whole farm, this build will not work for you.
Against champs like Teemo though, you have to be able to be patient. You need to lure him in or get a gank to kill him before you can farm at will.