his laning is pretty trash against basically every meta top laner. Its kinda awkward they're pitting him against Darius for lore reasons, but Darius should dumpster garen without any effort.
max Q because E scales more with garen's level rather than the skill's level, and it grants a longer movement speed buff which helps your shitty mobility. although i find putting 2 points into E helps clearing waves a lot. Q max also allows you to do your damage and get out, where E max makes you have to stay and take damage.
always try to use Q as an auto attack reset.
take grasp and all the runes that help you stay in lane longer.
i hear the hot thing is to build trinity force now. either way, i recommend something with CDR. playing garen without CDR feels awful.
above all, remember that you are Garen, the king of top lane. Your team answers to you and you answer to nobody. If you see a chance to assert dominance over an enemy, do it.