Some quick pros/cons to this build of Renekton
Pros: Good Mid-game/End-game Damage
Great dps/cc in team fights
Good survivability

Cons: Weak early game
Need knowledge to last hit minions

The reasoning for the weak early-game is, if played correctly, you don't blue pill until 1850g. This is crucial for rushing your BF Sword for the good damage.

These are the runes I'm currently using and are probaly the best for this champ.
Marks - Armor Pen
Glyphs - CDR(cooldown reduction)
Seals - Health - flat
The reasoning is, the health runes are good for survivability early game that you will need to stay in the lane to get the necessary gold.

I picked up a 21/0/9 build.

Rush the BF Sword unless you have to back before 1850g. Attempt to not have this happen but, if it does, get a boots of speed instead then save up.

This is your rotation - R>W>E>Q

Really this is up to you. I picked Ghost/Ignite because this seemed to work the best for me after trying out a variety of things.