Against garen, Garen’s kill pressure in the early game can be difficult to handle if you’re playing as a tank. Respect his all-in and try and avoid trading with him unless his Q is down. 
 
At level 6, Garen will gain access to his Ultimate R which can be used to execute enemies. Do not fight him when low, and definitely recall if you’re below 50% health as he might look for a trade. It’s better to be safe rather than try and fight and give him a kill.
 
Once Garen has completed his first item, he will be more obnoxious and looking to trade more and more. Keep this in mind and do not commit to fights with him alone.  Against garen, At level 6, Garen will have access to his Ultimate R which can be used to execute enemies. Do not fight him when weak, and certainly remember if you are below 50% of health as he could look for a trade. It is better to be sure rather than try to fight and give him a murder. Once Garen has finished his first article, he will be more heinous and seek to exchange more and more. Keep this in mind and do not commit to fight with him alone.