This is my first Renekton guide, it works very well at all. With the huge number of Healthpoints you get tanky enough to fight as long as you could in a teamfight.
Later on with the massiv movement speed you can run away(dont need :D) or with the number of ad you can easily kill the ap or ad carries.

Greater Quintessence of Vigor x3

Choose between this two Marks:
mid/late game: Greater Mark of Desolation x9
early/mid Greater Mark of Strength x9

Greater Seal of Evasion x9

Choose between this two Glyphs:
late MR:Greater Glyph of Shielding x9
early MR:Greater Glyph of Warding x9

With this masteries you get tanky enough and with the 10% armor pen you are making lots of dmg.
The Offense skilltree gives you a nice AD boost and the CR your skills will be able faster
But with the main tree "defense" you get a nice amount of health, lots of resistence and damage reduction.

I used this Itmes because i made greate experience with it.
First by boots and 3 health potions, after that get as fast as posible Trinity Force.
The Brutalizer and Vampiric Scepter gives you a nice boost of armor pen and a bit lifesteal.
after this two items you can buy a Warmogs and The Bloodthirster.
If there are more Ap then AD buy Force of Nature first and then Randuin's Omen

You can choose between the Stun and the Heal, if you want more damage take the stun else for faster leveling you should take the Heal and Slice and Dice

Renekton is a great teamfighter, you can jump with Slice and Dice and stun the enemies AD or AP carry. You can also initiate easily with Renekton and take out the Damage-Dealer.
If you are in the fight try to save your own Damage-Dealer.

Note:
If an enemy is trying to run away Renekton can easily jump over walls(like the Baron one) or
if you have to run away(but you dont need).