Garen was the first character I got decent with when I first started, and only have recently come back to grinding with him. This build emphasizes on your AoE damage and your ability to topple most opponents out of lane. Also makes Garen great initiator and gives him decent damage output with great sustain. I have won majority of my games with him and I highly recommend trying this build out at least once.

I went with starting out Garen tanky that way he can sustain until 6 and also have a major advantage over most top laners. Sunfire cape also heavily boosts your E's burst damage especially at low levels to give you that extra edge to get an extra kill or two. Which also helps with CS and damage steadily grows. And the further reasoning of Warmogs Armor is because it compliments Garen's already boosted Health Regeneration, at later levels giving you a invincible status if you watch your health and retreat at the right time.

You start out with Q to give you a pokey image, with a higher damage on first strike and speed, to get away. Than start to ramp up your E's power so that you can mix in Q and E for speed boost and bursty AoE to also help you annihilate runners. You may also use Q to give you an faster way to take down towers especially when you get damage/armor penetration. W has the passive of giving you extra armor + magic resistance. Which you can also activate to give you a decent percentage of damage reduction for engaging or dueling out in a 1v1 to make sure you can tank the damage.

Late game is usually always a tricky thing to talk about depending on outcome if your team is doing poorly/ who is fed. If you play it safe and watch how the enemy is building (Heavy AD or heavy AP). A good tip is to see what characters have the most kills I usually go top 3 and depending on which it weighs out to depends on my choice of final item. If I'm fed I usually buy another AD weapon to get an extra edge. If you have any questions I'm happy to answer them.