Hey Mobafire,
since I've never made a guide here before and just found a good way to play Renekton, I thought I should share my knowledge with you.
If you vote up or down, please leave me a comment telling me why. :)

Pros

- Great Ganker (Slice & Dice -> Stun)
- High Damage Output
- Good Lifesteal
- PERFECT Farmer
- No mana, but improved skills with >50 Fury

Cons

- Low Survivability
- Very vulnerable to CCs
- Late game hero (This can become a Pro if you play him right)
- Very item dependant

Reign of Anger (passive)

Renekton gains 5 Fury per autoattack. Having sufficient fury empowers Renekton�¢ï¿½ï¿½s abilities with bonus effects, but this drains Fury. Out of combat, Renekton loses 5 Fury per second.

This new system makes life such easier. You can spam skills when you've got enough Cooldown Reduction and pretty often in a team fight they're much stronger!
Cull The Meek

Renekton swing his blade, dealing damage to nearby enemies and healing for a portion of the life dealt. Fury Bonus: Heal amount and damage raised dramatically.

This skill is the one who let's you stay on your lane! Without healing, you get harrassed to death. This skill is also useful in teamfights because of it's AoE damage. Perfect.
Ruthless Predator

Renekton's next attack swings twice, dealing a portion of his weapon damage, stunning his target briefly and applying on-hits each time. Fury Bonus: Renekton's next attack swings three times, dealing a huge portion of his weapon damage, stunning his target greatly and applying on-hits each time.

A stun is always useful and double damage makes the difference in the end game. Triple damage? In the end game this can be the death for a caster. Let's say you've got 400 Atk.
400 x 3 = 1200
Some casters are dead by now.
Slice and Dice

Renekton dashes forward, dealing physical damage to targets along the way. If he hits a target, he can use Dice for a brief period of time for the same effect. Dice Fury Bonus: Renekton deals 50% increased damage and reduces the armor of units hit.

If you use this skill right, you can dash through all creeps, to the enemy champion and stun him with Ruthless Predator. This skill is also good for escaping. The only problem is it's cooldown.
Dominus

Renekton empowers himself with dark energy, gaining size and bonus health. He deals periodic damage to enemies around him and rapidly gains Fury.

This skill can make the difference in every team fight. In combination with Fury Cull, you can easily get the low-health champs down to the half. It also increases your survivability in a fight. Nice, huh?

Exhaust

This skill can get you an easy kill. Exhaust the one your chasing, Slice and Dice him all the way, including Cull of the Meek. Didn't kill him before it weared off? Ruthless Predator is your stunning friend.
Ignite

Trust me, you'll hate the ones who heal faster than you do. This skill turns the tables.

Red for ArP to increase your damage, Yellow and Quint to increase your survivability and Blue for CdR to get rid of that high cooldowns.
Do I really need to explain more? :)

A pretty basic 21-9-0 build. If you want, you can put more points into your defense, but keep in mind that it will reduce your damage output and so the healing.

A Dorans Blade and a Health Potion in the beginnning to stay as long as possible on your lane.
Ionian Boots to get rid of your high cooldowns. In combination with runes, this can make a huge difference.
After that a Vampiric Scepter to increase you Lifesteal and of course you turn it into a Bloodthirster. Try to get the B.F. Sword ASAP.
Now you get a Dagger to heal faster and deal more damage per second. With a B.F. Sword you turn it into a Black Cleaver.
A Sunfire Cape will give you some health at this point and even increase your AoE damage! (Thanks to B34stM0d3 for this suggestion)
Now, to stay alive, you should get a Guardian Angel. You will die pretty fast late game if you get stunned.
Now you can sell your Dorans Blade to get another Bloodthirster. Just improve the stats. :)

If the game even lasts this long, you should kill a lot of enemies with your massive AoE damage.

I'm focusing on Cull of the Meek to stay on the lane. Since you don't have much health, this also let's you stay alive in a fight.

The second skill I'm concentrating on is Slice and Dice. Why? The cooldown. You'll hate the moments where the enemy gets away with ~50 health and this skill is on cooldown.

Getting on point on Ruthless Predator to get a stun is important. I'm improving it at the end, because that's the time where it makes a big difference.

Dominus of couse on 6, 11 and 16.

You want to get some creep kills, but the enemy is doing a great lane control? This is the way you get their creeps:
- Slice and Dice into the mid of the creeps (make sure you hit atleast one)
- Use Cull of the Meek
- Dice back to where you came from
- ???
- Profit

In a teamfight try to focus on the main damage dealer (stun him if possible) and keep using your skills whenever they're on cooldown.

With this build you will get a lot of damage output AND Lifesteal, making you a great danger in every teamfight.

Thanks for reading this guide, I would like to see some comments and if you have some suggestions, tell me. It's my first build. ;)