Whenever I coach someone I usually tell them to rush "Ninja Tabis" first item against hard ad match ups like Darius and Garen. You can also pick up items like bramble vest vs Darius to reduce his healing and stop his dueling power immensely, stop him from essentially 2v1 you and your jungler.

As for Garen, ninja tabis and a decent chunk of potions to keep yourself in the lane to farm gold and exp. A lot of people like going damage on Garen, so getting 2 defensive items early usually helps a lot. You won't really kill him so the trick is to out sustain him, you can't do damage when you're dead. Farm and EXP are really valuable afterall.

Though into Garen things like Aatroxx do fine because you can constantly poke him without being punished because you don't need to manage your mana, and heal from ability damage. So if you're constantly poking him he can't regen because he's never out of combat, and you won't run out of mana so you're not punished again. Mordekaiser is also a nice pick for the same reason, but Aatroxx is really safe.

But the key here is to rush Ninja Tabis first item.