I would ask you to pardon the hyperbole I use in this build. I just really enjoy using this build so much and wanted to immortalize it/ share it with the world. I hope it gives you some ideas for items you might like with Garen.

I will finish this later.

Garen starts with an excellent early game harass that can be used to keep the enemy at bay. Without any items in attack or speed, Garen can keep ahead on early harass with his decisive strike while farming minions. Further, at level 2 and 3, Judgement is a good way to bring the enemy champ to less than half health. It deals frightening damage while giving you the ability to harass over minions and jump back over them for an easy escape. The goal is to use your basic attacks on minions and your skills on the enemy champ. It is important that you get 350 gold and push to the enemy turret right away for as early of a heart of gold as possible. In order for this build to work, you need to aggressively seize the gold advantage. As soon as you get 350 gold and push the turret, recall, buy the heart of gold and warp right back into the fray. This will give you a money item while maintaining farm advantage.

Next, you need boots as fast as possible for insurance from ganks. If you are halfway to your turret, you should be safe from pretty much any gank especially after you claim boots.

Judgement scales so well with attack that Garen naturally has the power to harass down most top champs. The problem is, he does not have any innate crowd control, so people have a tendency to slip from his fingers, especially since this build does not include exhaust or ignite. To make up for this, you will want to prioritize farming until you get a frozen mallet. Frozen Mallet not only gives the slow, but a lot of health which is useful for his passive health regen and for Atma's Impaler later. The slow effect will make it extremely hard for a lone champion like top to escape DEMACIAN JUSTICE. Also, once you build frozen mallet, it is time to push and possibly dive to take top turret. If you're winning farm, don't worry about a response gank; Garen is too escapey to get caught after you build the frozen mallet. After you take or lose top turret, it is time to start using your teleport to push the other lanes. Wherever there is a team fight, they need their Invincible Banox.

Atma's Impaler and Force of Nature are the two targeted resistance items. If the enemy is dominating with attack damage, you will want Atma's Impaler. I choose Atma's over other armor items because it gives Garen everything he needs: attack based on his health (which is helpful thanks to the Frozen Mallet, and it gives critical strike which increases the damage of his Judgement which can land critical hits.
Force of Nature is my preferred item for magic resist because it perfects Garen's passive health regen and gives him even more potential to a. get away and b. run balls deep into the enemy's business.

These items aren't really needed until the end because they don't add to Garen's invincibility. Invincibility is priority number one and after you have achieved it, then you can worry about more attack. Frankly, Garen's attack damage is already high enough with Atma's and Frozen Mallet. Also, many of your kills will be from stealing from the AD carry with Judgement and DEMACIAN JUSTICE.

If you have this build down, you will literally be able to run into a horde of enemies and get away fast enough for your team to converge on them like a hammer and anvil. If you time your Courage before crowd control gets focused on you and you use Decisive Strike right after, you can get away from anything short of a blitzcrank or darius pull. The goal in team fights is to wade into the deepest nether regions of the fight and spin until people die. Another way to initiate a team fight is to get them all to chase you and let your team converge on them and wipe them from the face of the earth.