This champion is one of the strongest assassins in the game, if they let you snowball you can dominate the game. Rengar has a huge pick potential past level 11 since his ult will be on a low cooldown and the damage he brings to the table is ridiculous. Add me and check out my stats on my account on EUW which is called WeREgyptianWomen. If you have I have carried single handedly 3 losing lanes. Rengar has great pick potential with his ult especially at level 11 or after. Even versus really tanky teams you can easily one shot their adc.
BEWARE THE PINK WARDS!!!

Ferocity control, comes with experience, You will need: 5 points in ferocity, your ultimate, and a Tiamat/Hydra. You may also want a Youmuu's Ghostblade

Step one: Ult with 5 ferocity and use your Ghostblade (If you have one. It's not necessary)
Step two: Press Q BEFORE you jump on to your target
Step three: While you are in the air (this is vital), use Tiamat (if you have it), E and W. Once you land, mash Q until they die.

Now the target is dead. YOU MUST STAND AND LAUGH OVER THEIR CORPSE FOR BM.

Mostly early game, or when you know that your damage will not be enough to kill the guy (could be that they are too tanky or that you dont have enough items to kill him with the combo yet).
This is when you ult then empowered E and snare them for the team to follow up with damage and assist you in picking off someone.

Farm till you get 6 avoid fights, and tax a lane lightly if you run by it, since you will carry late game. If you see oppurtunities for free kills on really over exteneded lanes go get a kill and if a one of your laners backs or dies hold the lane and take as much farm and experience. You do not need to make risky plays before 6, basically just stay safe get the scuttle crab for vision for your lanes and farm.

You find picks any time you have your ult. Bot-lane lane ganks are super effective, especially if they have an immobile champion. You can also gank mid or top if you see that you can easily combo with your laner or they have a lane playing really aggro.
Late game just look for people who split alone and 1 shot them mostly look for the adc.

Youmuu's Ghostblade is a one of the best items because it grants us many good stats for a cheap price. Rengar's job is to assassinate squishy immobile champions such as Jinx or Xerath who are known as carries. Youmuu's Ghostblade provides you not just the AD but the armor penetration, which is the most important stat for rengar to have according to how his Q works. The cooldown reduction it gives is also very nice. Another bonus, the item grants us an active which increases our attack speed and movement speed for a short duration. This allows chase, duel or disengage more efficiently and the active also synergises with Rengar's ultimate Thrill of the Hunt, to gap close faster with the youmuus movement speed active, before your Thrill of the Hunt runs out.

Tiamat is your bread and butter item on Rengar. Tiamat provides really good jungle clear, especially for rengar since he is slow at clearing camps and is not as easy for him since you will get chunked from the jungle early game. That is also why you buy a lot of potions with your early backs to keep you healthy and safe in the jungle. Tiamat also provides additional burst potential with works well with Rengar in general since he's all about burst damage.I specifically put Tiamat instead of Ravenous Hydra, to state the hydra is not worth rushing, Tiamat will provide you with all that you need to start snowballing the game, you build it into Ravenous Hydra only after you get your Youmuu's Ghostblade, Ionian Boots of Lucidity, Last Whisper, Ranger's Trailblazer - Warrior

The more you play him the more you will get better just dont give up! I made this account just to play Rengar on it and I had no clue how to play him. Now I'm one of the top Rengars on EUW and I got to diamond with Rengar. Practice makes perfect, later when you get better you can easily perform your combo. More or less smash the keyboard and win.