The first thing you should do if you want an agressive start is to take red buff with help of other champions or invade red buff enemy jungle, obviusly because he don´t need mana, and try to get first blood. So you level up Savagery and then bola strkie to avoid them to escape. next you will go into jungle and kill golems, ghost and wolves when you have enough money go to base to buy.

The items that i show at the beginning are for have a good speed that for rengar it is very necessary for his ganks, potions for restore our life to dont go every time to base. cause rengar dont use mana its very easy to move around the map and avoid base. The next item its is berseker´s greaves, it will give us the speed and attack speed to rengar.
i choose the Bonetooth necklace no only because its recommended its because if you get nice ganks at the early game it will be very helpfull, nest item is bilgewater cutlass i like this item very much for ganks cause its just perfect for our purposes, then and the faster possible bloodthrister with this combined with your jungling you can build it to get fast the 40 stacks for a damage of 100, this to give you the life steal that could heal you faster and and sustain heavy fighting against tanks, finally the infinity edge with its passive of 2.5 of damage when critical and 25% of critical chance and its usefully 80 of damage closes a perfect build to endure and win any fight.

In early game i recommend this before attack the enemy make sure you have 3 stacks of fury, cause with this you start with Savagery to to improve your attack then you will got 4 stacks, use immediately battle roar to heal a bit and do damage with this you will have fury completed and you use again battle roar, i try do not do bola strike cause i save it to used if he/she try to escape, this supplemented with bilgewater cutlass give a perfect gank.
the ultimate thrill of the hunt i use it just for two things, reach to the enemies and escape because with this invisibility can easily avoid any champion.

with his not need of mana or energy its very easy to dont´have to go base

have a nice combinations of abilities to ganks