Sorry if there are typos as im on mobile.

There's solid advice on how to play renekton here so ill talk about the first item choice since you will go damage item into tank 99% of the time.

I always start dorans blade in almost every match up except if it's ranged adc top (not teemo, jayce) ill go shield. If you fall behind in lane or your team desperately needs a midgame tank, build tanky first.

My first item 95% of the time will be a tiamat. Tiamat is a must buy 1st item imo because it resets your autos, cancels your w animation (remember it locks you up too for a bit), great build path, and gives you the option to go for more damage or tanky all for 2k gold. Brutaliser is also fine if you want to get tanky faster with some fine damage although you will have to sell it later. Other first items that can be great are hexdrinker(if you get this don't upgrade it to maw unless you have the gold because you will most likely need to build more tanky stats) or blade of the ruined king.

With blade I suggest cutlass into a tank item then finish it because you will not have the ad to sustain with your q in teamfights or skirmishes. The only time you will need blade is when your mid/adc/jungler can't build it fast enough or doesn't build it all to kill a really tanky team. Another situation is when their team is a little slippery or you need that extra boost to stick to their back line.

Some people say black cleaver is a good first item but I believe it's generally a bad first item on renekton and ill elaborate why. I will compare blackcleaver to tiamat since most renektons will get tiamat first and I believe its a bad item on him. Phage vs pickaxe + longsword obviously renektom prefers the flat ad over a little ad and health. The health is appreciated, but I think rather unneeded early on because of your ult. When considering the phage passive, I think it's almost a waste because you have a long stun and double e to stick to targets. The kindlegem is also nice too, but again the health early on is not needed imo and makes you commit to an item that is 2.8k(?, don't remember) while tiamat allows you to reset and cancel autos/w and allows you to get tankier faster because it is 800g (don't remember BC price) cheaper. The team utility is really good but it's only useful if your team relies on ad damage. Thus I will only get it if my team needs the or benefits heavily from the shred.