Renekton is the first champion to use a resaource known as fury. I'm not going to explain fury since his info in game is very well done. The key difference between a good and a bad Renekton is how they use their fury and ult. Learning to use fury well does take time. Sometimes you have take harrasment so you can get fury from the creep wave for a gank. My suggestion is save your fury for the stun. The damage is massive and the length allows for allies to run, catch up, or kill. If you are in the middle of a team fight use the fury on "Call of the Meek" to damage more and recieve more health.

Armor penetration is a physical carry's best friend. Early game it will chew through a doran's shield like butter. Mid game it allows what damage you are putting out to stay resonable even againt a fed tank. Late game all runes are practically useless and it has become based on masteries and items.
The flat health seals I take so you can take some harassment early and be okay. Their usefullness dies quickly.
The magic resist glyphs are for those annoying mages early game. So you don't get owned by two spells and have to hide behind minons forever.

Yes I know this is Phreak's masteries build but it works. If you don't like this and need more defense try a 14 offense and 16 defense build: Renekton: 2v1 or 2v2 part 2.

This item build will work if you are patient enough to get bloodthirster as your first main item and not jump to another item. Yes you will upgrade your boots to the cooldown reduction boots, but those are not selectable in mobafire as of yet. After you get bloodthirster if you need more health follow the build as is or take phage instead of brutalizer first. If you get to extreme late game sell doran's shield and take the recurve bow for madred's and upgrade when you can.
If in mid game you are having cash problems buy the health and attack speed potions and just save after that. Remember if its mid game and no ganks are happening run to the jungle and grab the lizard and golem buffs. This will help your ganks and your cash.

Renekton is a superb harasser for a physical carry. At the star of the game you will be right there with your melee mions and run to the center of the enemey minions and use your Q spell Call of the Meek. If you are getting harassed too much doing this just use it on the melee minions and hide behind your casters. Remember to run up and last hit minions that you can so your cash will flow well. At lvl 4 or 5 you should have enough cash for your boots if you have gotten a kill. If not its time to kill. Make sure you and your lane partner or jungling teamate are around half health or higher. Use slice(E) to get RP(your W spell) in range, then use Q(CotM) and E again to get in front of your enemies. If they aren't dead exhaust the one about to get away and kill the one with the least health. If you are lucky you get a double kill with your teamate.
Once you have your boots and vapiric scepter you are entering mid-game. Now your ult comes into play. Activate your ult around 75% health or 50% health. Stay near enemies so you can damage them with your aoe. If you are aiming for one specific target to kill use fury on your stun (W). If not then (Q). Remember your E can also be used to get away. If you are running and its only 1v1 you can dash into them and them dash again. This extra distance travled has saved my *** way too many times to count so don't forget it. Also during mid-game run into the jungle for those buffs and it keeps the enemies wondering what you are doing if you aren't in your lane.
Late game Renekton is tricky. Talk with your team and see what they want out of you. Do they want you to be more tanky or just do more damage? If they say more tanky don't get madred's. If you are getting raped by a mage then get Force of Nature, if its physical damage get a Sunfire Cape. If they want damage get madred's as planned.

That's all I got for you guys if you guys don't like the build I'm sorry but it has worked for me around 20 wins in a row now just random solo 5's.