well, I'm not a Renekton pro, but have it on good authority that building him should usually go as follows:
Masteries are up to you, 21/9 or 9/21 depending on how aggressive you want to go early on.
Runes are apparently best as ArPen Reds, Flat Armor Yellows, MR Blues (either flat or per level, your choice) and Flat AD Quints.
For items, you always build according to the enemy team. Facing 3 AD champs? Rush a Sunfire + Ninja Tabi. Facing 2 AP with lots of CC? An Early Hexdrinker + Merc Treads seems to do the trick.
General item options for him beyond what I've already mentioned are: S Visage, Randuin's, Last Whisper, Brutalizer, and sometimes Blade of the Ruined King can pay dividends if you hit someone with a 50-fury Stun.