I'm currently of the mindset that Garen should just farm safely in the early game, cinderhulk early will work well with his E to clear waves quickly, and if you're hugging tower it'd help your jungler to gank more easily.
Naaaaaaaaaahhhhh
If it's melee, when they go to last hit, auto, Q and then spin to bully them.
You are a lane bully early and have to show the opponent how the game is gonna be played.
Save ignite and wait for a jungle gank to get the kill (please for everyone, if your jungler is on their way, just farm like normal or play regular, if you can rotate your positioning away from the jungler so the opponent will naturally start to position closer to the gank, and only go in when the jungler is ready or in position).
Get the early advantage and just continue to push it. Farm a bit if they are ranged to get a quick defensive item then go ham.
I usually build black cleaver first if I'm at the advantage and then go sunfire or visage followed by last whisper (or if you really have the advantage just buy a full IE).
Garen is a bully and you have to show the enemy you don't scare. Garen never scares.