I rush a Brutalizer because Renekton needs the extra damage early. Sunfire Cape is also a core item for Renekton. The rest depends on the game, but I have given a balanced sample build.

Movement speed is important on Renekton so if you don't have MS Quintessences I'd recommend getting Boots of Swiftness. Boots of Lucidity may be worth considering instead of Mercury Treads if the enemy team has little or no CC.

Warmog's Armor is worth considering in place of Frozen Mallet depending on team comps.

Since the nerf Atma's Impaler isn't worth rushing, but it's still a good item to have after you have your Sunfire Cape if you want more armor.

Consider Force of Nature instead of Hexdrinker/Maw if you're taking heavy magic damage.

Consider Bloodthirster instead of Last Whisper if the enemy team isn't getting much armor.

There are more item and build order considerations to make, but I have listed some general ones here.