The masteries are mostly offensive, Why? because most of the items are armor and hp items so you'll need the Offensive masteries, and same goes for the runes too. In order to be a tanky/beefy Renekton you have to be spamming your abilities constantly and because Renekton's abilities are close up AoE damage, you must to be in the middle of the fight in order for the abilities to be really effective against your opponent(s). Renekton has a really cool passive, His passive is called "Fury". What it does is when he gains enough "Fury"(50 fury) from doing basic melee attacks or using your ability, he goes into a rage mode or something like that and his abilities do more damage then usual which can lead to killing your opponent.

In lane with Renekton is really cheap when you get slice and dice at level 1 because no one can really hurt you when you dash in and out of minions(but make sure the minions are almost dead before you dash in because you want to kill the minions). In order for the dash to work twice, you have to be hitting an enemy with the dash so the second one can come up again for use, IF you miss an enemy target or minion then........ you are pretty much dead, unless the enemy is stupid enough to let you go.
When you have 50 Fury you can do extra damage with your slice and dice and also reduces the armor of units that are hit.

Cull the Meek is a close up AoE ability that can do a lot of damage if really close to minions or enemy champions. This move can be used to harass champions and kill minions in early game.
Cull the Meek also boosts up with 50 Fury and can do serious damage to champions when ganking. This move should be done in between "slice and dice" before you dash your way back to safety.
Cull the Meek also heals Renekton by the damage done to the enemy,depending on the damage you've done to the enemy gives you that certain amount of hp.

Ruthless Predator is a stun move used up close melee range. this ability stuns the enemy target for .75 seconds which is enough to do some serious damage. if you have 50 fury or more then the stun ability will stun the target for 1.25 seconds which is enough to kill the target.
This move should be done after you used your "Slice and Dice" and "Cull the Meek" ability that way you can do more damage and possibly kill your enemy.

Pros
1.lots of health and armor less chance of dying in team fights.
2.All abilities are based on his physical damage
3.can dash in and out of battles and minions

Cons
1.Must be in close to do damage with Abilities
2.Can be easily taken out by a mage(Ryze)
3.Can't really chase to kill

When in lane and farming you should hit some of the minions and then "Slice and Dice" in and out of there to kill the minions or you can add "Cull the Meek" in between too, to do more damage. The "Ruthless Predator" should only be used on cannon minions only so you can finish them off with your melee hits or you can do it the other way around.