Garen is a tanki damage dealing machine. His passive gives him a great sustain at early game and in early game garen is a beast at level 2 you burst down almost any adc down with supports help.

When your playing garen adc you must remember you have ZERO slowing abilities so you need a support that can stun or slow the enemy. My personal favorite sup is nami cause she can heal you and when she hit`s her bubble it`s a sure kill. Other good supports are well almost everything that can stun or slow.

Garen is kind of weak at late game if you don`t have any defense you still do a lot of damege but you got to know when to go to the fight or else you will die in seconds. And try not to first to the fight cause you will die with 100% let your tank engage or mid laner or anybody else.