Rengar is a target fire champion that specializes in taking out lone targets with his burst. Now i am not going to be one of those guys who says there is not a right way to play this champion because that is BS. There is a right way to play Rengar. This will be explained in further chapters.
Now the runes I take for Rengar I use for two reasons. One to maximize his jungle speed, and Two so his ganks are effective. First I grab a Greater Glyph of Alacrity to increase his jungle speed, and the added attack speed comes in handy later. Second I grab Greater Mark of Strength once again to increase jungle speed and to also increase early game damage. Third I grab Greater Seal of Resilience for the increased jungle survivability. Finally i grab Greater Quintessence of Swiftness. This gives Rengar better chasing ability and the ability to get from jungle camp to jungle camp quicker as well.
I take standard jungle masteries for Rengar. Nothing really too special here so I won't elaborate any further.
Now when i jungle Rengar I start out Boots, three pots. You can take Cloth Armor if you feel you really have too but you wont build it into anything later so I find it useless, also the boots give you a faster movespeed through the jungle. The pots are a necessary item to live through jungle. Then I grab a vamp scepter to increase his sustain in the Jungle. Then i like to grab the Rengar specific item the BoneTooth Necklace because it has a passive which helps rengar a ton. Then grab a Phage because it is beneficiary to His Q which scales off of your AD and his W which scales off of health. Now you want to finish of those boots of speed into Beserker Greaves to compensate for Rengar's naturally low attack speed. After that you will want to Finish up that vamp scepter into a bloodthirster. At this point you will Finish up the Phage into a Frozen Mallet. After that you will buy your Pure tank item Warmog's Armour. After that round out your build with another blood thirster and you will be virtually unkillable.
When you jungle Rengar you can start either red or blue, it depends on what you want whether you want the early CD or you want the early AD. Honestly as Rengar start blue to make jungle clear times quicker and then get red that way you can have red for your first gank. From this point on as Rengar you'll want to give your AD carry blue. The Jungle path I take as Rengar is Blue, Wolves, Wraiths, Red, and then from this point you can A go back to wolves or B go back to base and buy the vamp scepter. It wont really affect your jungle to much and id honestly just go back and give the jungle time to reset. Then after that you will want to go back and as soon as you hit level 4 gank. Now there is two key things about Rengar's ganking. The first is his passive which allows him to leap at an enemy with a basic attack while withing a bush. The range on this is 600 the range of about Caitlyn. So if an unsuspecting enemy is roaming a little to close to your bush leap at them for the element of suprise. Next is his ferocity stacks these give each of his skills special abilities before you gank I suggest getting either 4 or 5 Ferocity depending on what you plan on doing. Depending on the situation you want to use either your Q or your E first when you gank. If the lane you are ganking has a stun or a extremely good slow and they are pushed then I would use your ferocity stacks on your Q for the insane damage output it provides, however if the lane you are ganking does not have a stun or a slow this normally means that they will have some nice damage output in which case I would use your E to snare the target in place. This will almost secure a gank for your lane and get you an assist.