Renekton, is a really fun champion to play, and once this build is complete you will top over 300 damage, and have over 100 armor, and MR. the skilling order may seem odd to you, but in all honesty his ruthless predator is useless early levels considering you basically stun yourself for that 1 second or so, plus his q is easier to harass with. His q at lvl 2 will give you enough hp to lane until lvl 6 and beyond considering you will continually max out q before any other skill.

For runes, i pick the basic arpen marks for dps. Dodge seals for survivability in the lane, cooldown glyphs, because this crocodile needs it, two hp quints for more early hp, and one arpen quint for more dps. I mix up the quints because renekton doesnt need that much flat hp added considering with this build you lvl up q and your going to be getting a lot of health back early on with that skill.

Basic 21/9 dps Masteries taking nimbleness in defense to compliment those dodge seals, and the cooldown in offense to give added cooldown on top of the glyphs

Firstly dorans shield, dont ask questions its perfect for renekton, 120 hp, 10 armor, and the 8 hp/5, lane until 1.5k or 2.3k, 1.5k for one heart of gold, and boots, 2.3k for two hearts of gold and boots, i get two for the extra health and armor, and the extra 10 gold per ten seconds is a plus. then Last whisper, gives great damage output, and can chop up tanks. Following last whisper i get hexdrinker, gives mr and some good damage, while giving you that hefty shield against some heavy ap.following hexdrinker i sell dorans shield and follow up with a bf sword, then get enough gold for bloodthirster. After that Depending on the team i either get Force of nature if they are more caster oriented or randuins if they are more dps oriented. Topping at over 300 damage and over 100 armor and mr you will be damn near unstoppable

EARLY GAME: Start with dorans shield, if solo top is available take it if not just go bot, put that point in cull the meek. Last hit as much as possible, around level 4 is when you can start assassinating people, Slice in , cull the meek, ruthless predator, dice out. A great lane partner with renekton is a really good blitzcrank.
MID GAME: by now you should have at least your heart of golds, and possibly last whisper, ganking lanes is a must, when coming in to the lane you want to gank landing that first slice in is a neccessity without it you may not be able to close in and land your ruthless predator.
LATE GAME: This is when your build is normally complete, and you should basically be unstoppable. using the normal slice in cull the meek ruthless predator dice out combo you should be dropping squishies in no time. Remember though you are not a tank, let your tanks initiate and slice through their tanks to get to the carries/squishies.