Renekton needs AD to support 2 of his attack skills. But besides that you do want max CDR because his skills require NO COST!!! However, this is not that easy with an AD char unless you want the active of Youmuu's (which I don't). So the boots, brutalizer and stinger max CDR (38% CDR) and they do it very cheap so very fast giving you AD and AS at the same time. Upgrading both items is very low on my list so leave it till u got all other items.
Next we focus on more AD. Black Cleaver and Bloodthirster will do fine. Whit's end can give more AS with MR. Or choose Last whisper for more AD and AR-PEN.
If you have more funds and the game lingers on finish Youmuu's and Nashor's tooth.

We do not invest in defenses much. So make sure u have your ulti up when you engage (its CD will be around 60 secs with 40% CDR). It gives you all the defenses you should need. If its not up you are very vulnarable but you have an excellent escape with Slice and Dice. So just jump in, get some hits and jump out.

The CD of his skills go down as the level of a skill increases. So its better to focus on two skills only in your skill sequence (Q and E skill) to get their CD down as quickly as possible. Just put 1 point in the W skill and level you ulti as usual.