Renekton has a powerful harass combo with his 3 main skills. You should focus on last hitting creeps in top lane and then ripping off his harassment combo when ALL THREE of your base skills are off of cool-down (obviously waiting on E the most, it has the longest CD). The idea behind Renekton's harassment is that you should try to pull it off and get out before they can retaliate or do any damage back to you. All this is possible because of his overpowered slice-n-dice ability [E].

Stay passive until level 3, and then here is your bread & butter combo for harass:
1. slice in (E)
2. auto-attack
3. ruthless predator (W) [resets the auto-attack timer, so occurs immediately]
4. auto-attack
5. cull the meek (Q)
6. dice out (E)

There are other variations that use Q first or earlier... in the hopes that it comes up again before dicing back out or running out. Obviously... when you want to kill... you do not dice away, but you use it to chase even further, and you should probably pop exhaust on the target too.

If you have successfully pulled off this harassment twice, you should be looking for the opportunity to do a third time... except this time use exhaust & stay in there for the kill. Be ready to use slice-n-dice for chasing, so if they let you close range without it... they are practically giving you the kill. It should be worth noting that you should try to always slice across an enemy with the first half of slice-n-dice [E] otherwise you don't get to use the second half (dice). If chasing, use ghost to ensure that slice lands so you can use dice. : )

The name of the game is armor pen. It helps Renekton's attacks and his 3 main skills. Stack damage and armor pen. The phage is just for that extra health & chasing power. Pay special attention to how Renekton's ruthless predator [W] works; it hits 2 or 3 times and applies effects each hit. So phage & black cleaver are processed each hit. Don't worry about upgrading the phage until your very last thing. I would upgrade the brutalizer first.