There's not alot to say about this. If you have ever played Garen you know that it is really easy.
You go in with 'Q' then when you start taking dmg you immediately use 'W', and when you get close to the enemys you use 'E' and spin. When you see an enemy is low use ult on him. If he does not die use your 'Ignite' to finish him off.
A little tip: If you are sure one of your carry can get the kill. Try to use ult a little bit early, so you will leave the champion with 5% health left. Easy for your carry to grap the kill.

Only thing really to note is: When you play vs Riven, Teemo, Jayce or champions alike Teemo and Jayce. Max your 'Q' first. Because you will probally not get in range to use 'W'. Which also give you the ability to silence. Which is essential vs Riven.

Remember that Garen is a brute tank. He can tank but also do alot of dmg. Sometimes the best defense is a strong offense. He is not an early, mid or late game champ he is really powerfull throughout the game. The only thing that really changes is his ult in higher levels. Where it will do alot more dmg.