I have two builds here. Both have been tested a bit, although not too thoroughly. He IS a new champion after all... I've created both builds similar up until the boots, so that hopefully at that point you'll have an idea of what build you need to use. (Or sooner than that hopefully...)
So let's dive right into it.

These guides assume you already know how to play Renekton. If not, go take a look at the champion spotlight. He goes over plenty of tricks you can use as Renekton, as well as spell combos. I've found that Renekton can NOT take middle lane. So don't even try. That's just what i've gotten from experience. He can solo a lane (Providing it's another melee champ if you're lucky) but he also makes a GREAT partner for his stun, and ability to chase.

Of course, you can play around a bit, find a build that suits you, and that's fine. My guide is merely a suggestion, it's also for my own use.

The first build is Magic Resist. Naturally, build this way if the enemy is prominently AP. Yes, you CAN have more/better Magic Resist items, but you'll notice that Renekton relies on being able to use his abilities fast. That's where all of the cooldown reduction items come in. You can stun an enemy, and REALLY wail on em, making them regret attacking you.

At about lvl 4-6, you can start harassing the enemy in your lane with an E,Q,E combo. If you have time, You can try E,Q,W,E. The W attack of course stuns the target, preventing them from leaving your onslaught. You may not be able to kill the enemy until lvl 6 unless they are a squishy champ, but you'll definitely deny them exp and gold, which is worth it in the long run.

Naturally, in the beginning, try to ration your potion for as long as possible, using your Q ability to heal while fighting minions.

Late game, you'll be annoying the AP users as they struggle to do damage to you, all the while you are beating on them, and stunning them so that your allies can catch up and destroy the enemy.

You have a Gardian's because... well... Renekton IS tough to kill, but he's not SUPER tanky. This is insurance.

This build is my favorite so far. More hp, same 40% Cooldown reduction, Better farming ability, Aoe like it's no one's business. and more armor.

The armor build is of course, if you have many enemies going AD. Sunfire cape is a nice addition, especially when you get your ultimate. Aoe has never been easier when you just have to stand around the enemy. You'll want the cape early game to keep your lane enemy at bay, and kill as many minions as possible. The faster you get gold, the better you can fend off multiple enemies with your new items.

This build doesn't have as MUCH magic resist as the other build, but at least you have some. With the addition of your runes, it's a good fit. I prefer this build, but if there's casters giving me trouble, I'll be wishing I built the other way quickly. This build ALSo has less AD. Which means you MAY be attacking the enemy for a longer period, but with the slows ans stuns it shouldn't be a problem. If it IS a problem for you, i'd say get the Last Whisper INSTEAD of Guardian Angel. I know the armor/magic resist/extra life is hard to give up, but if you're already hard to kill, may as well buff up your attack.

This build ALSO contains a lot of slow, which, complimented by your W and E abilities, prevents the enemy from escaping your fury.

Well that's all I have to say for now. This is my first OFFICIALLY PUBLIC guide, so go easy on me. I hope someone finds this guide useful. I'll keep experimenting with Renekton and making any changes I think the guide may need. For now, This is what I got.

Good luck fellow crocs! Let's give em a headache! X3