I'd suggest grabbing Brutalizer early and building it into a Cleaver sometime in mid game. Even if you're building to be a tank, having a AoE armor shred to boost the damage of any AD champ on your team (ADC included) is too good to pass up.
Sunfire is very nice on Garen, but it's better built sooner than later. imo damage items *IE, MoM, Atma's* should come in the mid~late game build to keep Garen's damage output relevant (his high base damage lets him build his beefier items earlier).
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