This is a build I made for Renekton with the help of my friend while I was playing last night. I started the game thinking I was going to lane side for some support early game- I was forced mid as a little side note. I started as more damage output, but decided to make it more of a tank/AD build as the game progress. It may not be that great, but it is my first posted build, and my end stats were 20/5/4 in a 3v5 game. my team being the 3! And with that, I'll explain it!

Really depends on how you want to build him. I don't play that often and so I don't have a full rune page at all, it's more of a mix between damage and tank. I personally am going for AD/AS and some armor/magic pen runes for my first page for him, but you could also go Tank with some armor and magic resist runes. When I actually have both pages/ League is back up after the Karma patch I will through in the runes I used. The runes I have selected are what I think would be best used on him. I haven't gotten a full page like I said, and these can be changed based on what you think would go better. I'm always available for some insight and would love some feedback.

I chose more defense for the masteries as a type of tank page simply because I wanted him to have some defense while I built him AD. It may not be alot, but it helps. Especially early game where all the AD is being bought through the items. Masteries could be changed to more offensive based on how comfortable you are with your harassing.

For the Items, I start with a Doran's Sword, along with a Health Potion(which after this patch wont matter anyways, because you wont be able to get both). I usually go back to base at around 1300 gold and grab the Ionian Boots of Lucidity if turret is getting pushed, if not I would grab tier one boots then start the Infinity Edge. Based on how you do it, finish the Boots first, I find them more helpful with earlier game. However, if you want more damage first, I would personally grab the B.F Sword then finish the boots and finish the Infinity Edge afterwhich should be around Mid game- at this point I usually get The Black Cleaver for some AS and some decent Damage, then start building the armor and magic resist. Which should now be mid/late game. Now, these items are strictly based on the enemy team, if it's more physical with some magic, go Sunfire's Cape and Then Guardians Angel. However, if it's all physical damage, either get a Randuin's Omen or possibly a Frozen Heart. If it's more Ability power enemies, replace the Sunfire's Cape for a Force of Nature AND SWITCH THE BANSHEE'S VEIL WITH GUARDIANS ANGEL and save the Guardians for late game. At this point, it should be Doran's Sword, Ionian Boots of Lucidity, Infinity Edge, The Black Cleaver, Sunfire's Cape or Force of Nature, Guardians Angel or Randuins Omen/Frozen Heart/Banshee's Veil for your five items. If the game is still going on, start a Banshee's Veil. When thats finished, sell the Doran's Sword and buy The Bloodthirster or a B.F Sword to start it and get some damage.
so to recap:

START/EARLY:
DORAN'S SWORD
HEALTH POTION
to
IONIAN BOOTS OF LUCIDITY
or
BOOTS OF SPEED
B.F. SWORD

MID GAME:
finish INFINITY EDGE
finish iONIAN BOOTS OF LUCIDITY
THE BLACK CLEAVER

MID/LATE GAME-based on enemy teams:
Pure Physical:
SUNFIRE'S CAPE
RANDUIN'S OMEN/FROZEN HEART

Mixed Physical/Ability Power:
SUNFIRE'S CAPE
GUARDIANS ANGEL

Pure Ability Power:
FORCE OF NATURE
BANSHEE'S VEIL

LATE/END GAME:
BANSHEE'S VEIL/GUARDIANS ANGEL
THE BLOODTHIRSTER

Now this is personal preference, I prefer to start with his Q Cull of the Meek for some life steal and move on to his W Ruthless Predator at level 2 for a stun so that me and the partner laning together can get more damage out before he runs then get E Slice and Dice at level 3 for some damage in a small AoE or to flee. However, if someone jungles and your taking solo lane or by some mis-hap mid, start with Cull of the Meek, then get Slice and Dice so that you can escape if they start getting aggressive, then get W For the stun at level 3. From there I like to get Ruthless Predator at least to level 3 first then get the Ultimate Dominus at level 6. From here I go E+Q+W+E+R X2 then the last two skills are up to you. I personally get one more in Ruthless Predator and then Cull of the Meek.

Now, my personal preference here is Ignite and Ghost. Ignite for after dealing damage to a champion and they start running and Ghost is out or Slice and Dice doesn't get you close enough for a last hit. Gold is very good, especially early game for the Infinity Edge. Then I choose Ghost to either chase or run along with Slice and Dice for those as well.

Some other good spells, in my opinion, would be Exhaust and Flash.

As for team work, I also played a game with a tank and laned with a Renekton. In short, feeding Renekton is a scary thing and as the tank that game, I made it my job to do that. So, if you get a person that wants you to be a sort of carry, lane with him. He will make it his job to make sure you get fed. Always remember though, don't initiate fights, always let the tank go first. And since Renekton isn't listed as a carry, if you have a carry on your team along with you, and your both fleeing, save them over yourself because you are in a small way tanking first over fighting and that carry is going to need to be fed more than you by late game or you wont have as good a chance to win