This guide is not meant to be a "real" guide. It is just for quick information and not to be taken seriously.

At level 1 in any lane let the enemy champion/s move up near the middle of the lane then run in and E then retreat as soon as you stop spinning or you are getting low on health. This usually secures the kill for you at level 2 if they drop below 50%.
Level 2 usually means a kill, as soon as the enemy starts pushing away from their turret hit Q and go in for the attack, once Q goes off quickly hit E for that spin. Don't chase if they don't die.
The rest of the match you are pretty much doing the same tactics but you would need to activate W when going in for an attack just because it reduces damage done to you.
Remember Garen has an amazing passive that allows him to regen hp when he is out of combat so use it as much as possible instead of hitting B to go back.