Renekton is a very strong solo top champion with a great poke and great sustain. If you can utilize this early game and get fed enough you can even turn around a gank and maybe even score a double kill. I am going to try and describe to you as best I can as to how to do just that.

My choice in runes are as follows:
Armor Pen Marks - I use these as the early armor penetration allows you to get a lot more damage off, renekton has very high base damage so these are perfect for him as they allow him to deal a lot more damage early game (depending on your opponents first item and masteries/runes)
Armor Seals - These are just there to mean that the enemies poke is less effective meaning you can get the better of most exchanges and also to make you more resistant and more likely to survive early level ganks.
Magic Resist Glyphs - These are there for the same reason as the armor seals. They can help you get the better of exchanges in early game as well as make it easier to survive earlier level ganks.
Movement Speed Quints - These are here to make it easier to escape from ganks, teamfights and to help when trying to chase down opponents.

The mastery choices are 21/9/0 as Renekton is just Tanky and has great sustain anyway, so extra damage early game from them just makes his poke stronger. Although you take a couple of points in the defensive tree to help you with being more beefy early game and, as I stated with my choice in seals and glyphs, help you with early level exchanges on lane.

You start boots 3 health potions as that is never a bad idea, as the jungler will usually start with cloth armor and 5 health potions, this means that you could quite easily escape their form of CC by simply using your E and running off. This is also great as you could potentially have more movement speed than your enemy top laner, this means its also easier to escape if they seem to be outpoking you or going for a kill.

After that you would hopefully be able to buy two Doran's Blades meaning that you are more beefy and have more damage and a bit more lane sustain due to the small increase in lifesteal.

You then choose your boots, Either Merc Treads or Ninja Tabi, this is where the enemy team comp comes in, you want to vary your boots by judging whether they are primarily AD or AP based or if they have little or a lot of CC. For example if they have a lot of cc yet only one AP user you would still build Merc treads as you need the tenacity, also the ad damage gets lowered later as you build Sunfire cape which means that you get a lot more armor.
You would then build a Brutalizer as this adds a lot of damage and gives you armor penetration, combined with your armor pen marks this can mean that you deal close to true damage to your opponent.

After Brutalizer an item I find to be very useful is Sunfire cape, this is because it deals damage to enemies near you, and your ult also does the same, this means you've got two damaging auras when your ult is active and this really does allow you to win some fights as the damage slowly adds up. It also makes you a lot more tanky against the enemy carry and possibly the enemy solo top.

Then you could build a Frozen Mallet, this is a great item as it allows you to slow your opponent as you cling onto them dealing damage the entire time. This also means that your Sunfire cape is more useful as it deals damage whilst you cling onto them.

Now what you would do is start building a Warmog's armor, as you do this you would need to sell one Doran's blade to free up the inventory slot. You build Warmogs armor as it helps you become a lot more beefy so you can take more hits from the enemy in team fights. It's also very useful as you would then sell your other Doran's blade in order to get Atma's impaler meaning that you get a lot more bonus AD from the great amount of health you have from your Sunfire cape, Frozen Mallet, and Warmog's Armor.

For your final item you would sell your Brutalizer and get a last whisper, you would do this as you gain more armor pen and also more AD, this means you get higher damage in the late game.

(A possible difference could be to sell your Sunfire Cape and buy a Guardian Angel as this would give you armor, magic resist and you also get the amazing effect of Guardian Angel.

Your end items should be the boots that you chose, Sunfire Cape, Frozen Mallet, Warmog's Armor, Atma's impaler and Last Whisper.

Or: Chosen boots, Guardian Angel, Frozen Mallet, Warmog's Armor, Atma's Impaler and Last Whisper.

For poking the order you would want to use is the following:
Slice into the target, auto attack, ruthless predator, auto attack, cull the meek, and dice back out. this means you hit the target at least 4 times (maybe 5 or 6 if you can hit them with your slice and dice on the way in and out)
The reason I choose to level my abilities like that are because the main thing you need the W for is the stun, this means getting that first can allow you to poke effectively upon hitting level two when you get your Slice & Dice. You max out Cull the Meek first as it gives you a lot of sustain. When on full fury you should either attempt to poke, using the fury on either your Q or your W, or use it to clear out some minions by using your Q. The reason you wouldn't want to use your E to poke on full fury is because it requires you to hit your target with the second E first. The dice part of slice and dice with full fury would reduce the enemy armor but because that is the way you escape after poking and because it is the last ability you use when poking it is generally a poor choice to spend the fury on. The reason you max your E second is because when team fights start to appear you need to be able to do more AoE damage than single target, meaning that, except from stunning, the W isn't too great of an ability to max second.

You use Flash as it is simply a must have on nearly all champions, flash allows you to potentially escape over a wall, you can then cover more distance by using your E whenever possible.

You use Ignite as it allows you to counter enemy champions with high life steal or high health regeneration, such as Dr Mundo or Warwick. It's also very useful for picking up kills at the end of a fight.

Renekton is a good champion for low elo ranked as it you don't have to rely on ganks to take down your enemy top laner in most cases, this means your jungler can help middle and bot lanes or stay in the jungle for longer. It's a great champion to build up your elo with as it's rather easy to learn as well. Also once you learn how to play him effectively, you can easily change a gank on you into a gank on the enemy from usage of your ultimate. He is also very good in team fights due to his high AoE damage and tankiness, also, because of his ult, he becomes a bigger target so its more difficult for the enemy to target the carry with a renekton in the way. It also means you can tank skill shots that were aimed at your carry. Good team fights can win you the game/

Pros

Has a Gap Closer.
Strong AoE damage.
Isn't a mana user.
Has a very effective single target hard cc.
Is very tanky.
Can reduce enemy armor with the fury bonus on his E.

Cons

Can fall of late game.
Relies on snowballing early game
Can be countered rather easily.

When playing renekton in teamfights, you should play him as an initiator and a tank (if you don't have one on your team, if you do he is an offtank), You should aim to be in front of your AD carry so the enemy can't land skill shots as easily on them. You should also aim to be in the middle of the fight as you can cause maximum disruption that way by using your Q and E to damage all nearby enemies, it also helps to be in the middle as your ultimate has an AoE damage effect on it, and you also build sunfire cape, this means you deal decent damage just by being near the enemies. Also as renekton you should aim to stun the enemy carry whenever possible to minimize the amount of damage they deal.

This build causes you to become a monster in lane. You can exchange very well and also take little damage from enemy pokes. It also makes you more useful in team fights as your armor pen means that you can deal decent damage against any opponent with your AoE abilities, it also allows you to be very beefy meaning you can take a lot of punishment in the team fights. It gives a nice amount of damage too as you build a lot of health up with this build from building Sunfire Cape, Warmog's Armor and Frozen Mallet, so when you get your Atma's it causes you to gain a lot of Attack Damage.

As Renekton your early game should be based around farming and poking the enemy top laner to prevent them from getting CS. The faster you get ahead of your opponent the better. You should focus primarily on farming and poking until your team decides to set up some team fights, in which case you should remain with your team. A good ability to help with farming is Q, it's especially powerful when you have full fury and nearly allows you to one hit an entire minion wave if you are positioned correctly.

I hope my build achieves what I aimed for in making you become a very tanky yet effective renekton, even in the late game. I worked hard on this so I hope you found it useful.