I am a Garen main with currently 180+k mastery in Garen. I love Garen he has a great balance between damage and tank and i just enjoy playing him.

Pros
Great sustain
High damage while building full tank
Easy to play
Great level 6 all in

Cons
Can get kited easily by ranged champions
Very little cc

These are the runes I run you could can go scaling armor and magic resistance if you want but I prefer a bit more early game defense.

You can swap out unyielding for recovery to stack with your passive if you want.
You can also swap legendary guardian for swiftness.
Grasp is 100% the best keystone for Garen as it scales with max health and you build quite a lot of health on Garen.

I always start q it makes farming a bit safer and does more damage than your e early game.
You max e because it is your main damage ability.

Early game farming just auto like normal or to push use q to ensure you get the cs.
Later in the game you can e entire waves and kill all the creeps.