First think with renekton is to know how to lasthit creeps and a good support like sona on lane for better survival long time before 1st way to shop.Starting with doran shield will give u enought time to farm a bit with ur huge armor and armor penetration. Boots of lucidity and last whisper with ur talent tree and runes will give u total of 23.85 % cooldown reduction and total of 24 armor penetration ( plus 40 % of enemy armor ) thats pretty good to harras enemy champion even when some tank is nearby. Brutalizer will give u a bit more dmg and total of amazing 39% cooldown reduction. Like a melee Ad fighter almost in all cases will enemy champs att first you so u need to survive, Warmong will be the best choice for AD renekton it will give u in max stack up 1400 hp and a nice hp regen/5 sec ( u can combinate this with some life steal but i think in will be little bit silly :) ) and finally u can make warmong even more usefull with Atma impaler. At this stage of ur build u must have 1000% to finish the game in 5 minutes.. FF u can finish the game with Ghostblade will give u more cd reduction and better penetration ;) or u can buy it between warmong and atma ;) Last words: u have really more armor and hp with staked up warmong it dont show here ;) Gl n Hf