I have always think that you should personnalize your masteries and runes. Personnally, lots of ren are not using quints of swiftness but it piss me off when running beneath 400. I focus on offensive masteries because I tend to DPS, but for the first Item, since Ren has no mana, hight healt regenaration seems to be the right choice to stay in lane longer. But those are just a suggestion, tough it was tested and approve by F4G MAN.

Begin with ruthless predator, because it hit hard and the stun is really usefull.
Then get slice and dice to harrass or flee, if you are already low on health take cull the meek.
Then I use to up Cull the meek because of its low cooldown and healing properties. No need to focus Ruthless predator first, it hit already hard. Then I focus on Ruthless predator then finish with slice and dice.

My ren gameplay is more focus on DPS, but laning is the samething even if u will tank. Start with ruthless predator, then slice and dice to harrass easily. Then boost cull the meek to heal yourself while laning. harrass tips : slice into minions ruthless predator a champion, cull the meek then dice back. You will see that he is much more stronger than other champion so if you play against to defensive foes. Try to lure backing so they followed you, then slice to them, stun, cull the meek and use exhaust and ignite if needed. Even if Renek has great power in early game, he has almost no life so keep an eye out on your HP but stay aggressive.

Because of slice and ruthless predator, ren can do one man gank. Just go in a brush, wait for the best moment and go get a kill!!

If there is a tank try coming in after him, if not slice in and stun the biggest threat and eat it up! If you are focus, fake an escape with slice, use your ultimate and kill your ennemies! Seriously this can change the fate of a team fight.

I late game, you will have some choice to do after you have buy the Last Whisper. You choose to either
1-keep your sword of the occult
2-change it for another Bloodthirster (what I do most of the time)
3-or trade your attack damage for a Warmog's Armor
4-Or if your ennemies have no armor, you can trade either your last whisper or Youmoo's, but it does not happen often. And Youmoo's active is really usefull

I really prefer the second choice because you deal huge damage, you have a lot life steal, even if you are a bit squichy, your ultimate, combined with great life steal will save you.