Hey this is my first build ever ^^
This is how i Play Renekton.

Dmg Pen : i stack these because Renekton is a all physical Dmg hero (except for his ultimate which is magic).
Cooldown reduction : i stack these because of Renekton's high ability Cooldown, with less CD you get more survivability because if his Q spell.

its kinda basic...put offensive just do maxmixe the dmg as much as possible...and some in defense just for survivability.

so....heres the tricky part.
first off i start with a Doran's shield and a pot for the extra health/regen.
then straight of with the Cooldown recution boots for lower Cd's.
then phage to for the slow in the start.
Zealbecause its kinda cheap and gives a good attackspeed/Movement/crit.
phantom dancer (alrdy had a zeal) for the nice movement speed/crit/attackspeed + phage this makes a nice a nice chase combo.
then off to trinity force because of the extra damage after you make a ability.
now this is prob around mid/late game so if yyou have a hard time surviving teamfights/1on1 etc etc....i would suggest a bloodthirster because of the lifesteal....but if you are fine and just want extra dmg...i would go for black cleaver.
and to finish it off...with a spirit visage for the CD recution and some extrea health from Q ability and mby your lifesteal.

go with W at lvl 1 for the survivability if you get ganked in the bushes or you and your teammate on lane want the firstblood.
then spec Q untill its lvl 5, this gives you a good Hp return if you are getting harassed or in teamfights where there is alot rumbling!
now you are lvl 6-7 and should spec after how the game is going...if you need to chase alot i would suggest you spec in E and not as much in W because of the lower CD.

exhaust = slower MS/less dmg, works great if you wanna gank someone like trynda or someone else that does Quite alot of dmg.
ignite = if you are facing mundo/vlad you want to be able to kill them...+ it gives you a range DOT if the enemy champ outRunned you.

Just wanted to make a quick comment here before i finish this build/guide off.
this build has worked great for me!
some of you may think that these items or w/e is just madness...but ive played like this for my latest 8-9 games with Renekton.
http://img593.imageshack.us/img593/635/lolyn.png
i think it went kinda well. ^^
btw..dont mind the leave...that was only because my LoL stopped responding and i couldnt get back in -.-'' it somehow ''internet'' froze...but i could still see everyone else move xD