Garen is a good solo top with no mana, easy to play, but a bit annoying with his Demacia!, and for those who didn't notice, his Ult is only to finish enemy with low health, but at end game you can use it on enemy with only between half and quarter life (depend on the enemy's life and how much they can absorb the damage). Q and E canbe useful for chasing, Q for the burst of speed and silence, and E to pass trought minions. W can be use if you're low on life, been ganked or if you want to chase an enemy near his turret. This is my first build so if you want to help posting a comment, do it.

For farming with Garen, there is not much thing to know, just spam E and auto-attack (don't forget Garen don't need mana)

For chasing enemy Champion, use Q to reach them and silence them, then use your E (it does more damage than just auto-attack) and W for a greater protection (you can use your W when using your E), then, to finish the job when the enemy is low on health, Ult.