Renekton is an excelent dps but he needs to have some hp to protect him otherwise hes gona die way to easily.
Runes
as runes i would advise you to go all out dmg and atack speed get 9 greater marks of strength,9 greater glyphs of strength,1 greater quintessence of Strength,2 Greater quintessence of Alacrity, and 9 greater seals of alacrity.
Masteries
for masteries u should get all atack and one from utility which is the masterie for ghost

http://cdn2.mobafire.com/images/rune/greater-mark-of-strength.png 9 MARKS OF STRENGTH
http://cdn2.mobafire.com/images/rune/greater-glyph-of-strength.png 9 Glyphs of strength hhttp://cdn2.mobafire.com/images/rune/greater-quintessence-of-strength.png 1 Greater quintessence of strength
http://cdn2.mobafire.com/images/rune/greater-quintessence-of-alacrity.png 2 greater quintessences of alacrity
http://cdn2.mobafire.com/images/rune/greater-seal-of-alacrity.png 9 greater seals of alacrity

For items id say you should start off with
http://cdn2.mobafire.com/images/item/dorans-shield.gif
dorans shield is usefull cuz it provides hp regen and armor and teh health that renekton needs for the laning process but if u dont like the dorans shield u can also start off with
http://cdn2.mobafire.com/images/item/vampiric-scepter.gif
the vampiric scepter can be a usefull item helps you heal faster on minions but id advise u to get dorans
next on the list
http://cdn2.mobafire.com/images/item/berserkers-greaves.gifs
berserker greaves are excelent for renekton cuz they provide the atack speed that he desperetly needs u should stay on your lane if u can untill u have enough gold to buy berserkers
next item is
http://cdn2.mobafire.com/images/item/the-bloodthirster.gif
this item is excelent cuz it provides renekton with damage and lifesteal which is very usefull for him and if u start with the vampiric scepter u can turn it into the bloodthirster
then after i get the dmg and atack speed i go for hp and buy
http://cdn2.mobafire.com/images/item/frozen-mallet.gif
item is excelent cuz it provides renekton with hp,dmg and gives him the slow wich helps him catch his pray easier
then after geting frozen mallet i get more dmg
http://cdn2.mobafire.com/images/item/the-black-cleaver.gif
i get this item cuz it gives me atack dmg and atack speed helping renekton a lot
now with this item its all up to you.its a item that depends on if the enemy has a mage,dps or balanced i mainly go for this item
http://cdn2.mobafire.com/images/item/guardian-angel.gif
it provides me magic resistance and armor which is usefull if the enemy team is balanced which it mostly is plus its passive is rly good but if the enemy has a lot of mages or a fed mage i get this item
http://cdn2.mobafire.com/images/item/banshees-veil.gif
it gives me health and magic resistance and its passive helps alot against stuns and mages but
if the enemy team i mainly made outa dpses i get this item
http://cdn2.mobafire.com/images/item/sunfire-cape.gif
sunfire cape gives renekton the hp and armor that he needs helping me if they have a fed dps or a lot of dpses
for my last item i sell dorans or if you bought vampiric scepter then u just buy this next item
http://cdn2.mobafire.com/images/item/infinity-edge.gif
i get it because of the damage :D

for summoner spells i would advise
http://cdn1.mobafire.com/images/summoner-spell/ghost.png
ghost is my main spell that i use for him it helps me chase down a enemy or if needed escape from 1
for my second spell i use
http://cdn1.mobafire.com/images/summoner-spell/ignite.png
this is really usefull for killing enemies that can heal or for enemies that are escaping and have low hp but u cant reach em with your abilities
but you can also use
http://cdn1.mobafire.com/images/summoner-spell/flash.png
it can help you with escaping or
http://cdn1.mobafire.com/images/summoner-spell/exhaust.png
its usefull for slowing down and killing enemies
you can also use prety much any second spell but ghost is the main 1
http://cdn1.mobafire.com/images/summoner-spell/teleport.png

If u have a range heroe on the enemy team on your lane u probably wont be able to farm so i advise that u just hug the tower but if u have a melea be agresive and harass him with your q
also u can use the q to last hit minions and kill never use the q on 1 minion always go in a group of minions and do it there u heal there more keep harassing untill u get your ultie or if u have a good laning partner u could probably atack em at lvl 2 when you have your stun or at lvl 3 when u can dash at em but mainly wait till you have your ult then u know that u will survive the fight.in team fights always go in with your ult cuz u might get stuned,silenced etc
and not be able to do it during the batle use your q to hit all of the enemies and w to focus on 1 enemy also use your slice and dice to do a lot of dmg to the champions.