I generally choose a lane that I see is being pushed in on my side of the river and build my ferocity on the way toward the gank. I will open with the empowered bola, as a rooted laner is going to be an easy target for my teammate and I to focus down. Rengar is very good at destroying solo laners that push too far past the river. Make sure that the lane you choose is ready for a gank (your teammate isn't low on health, the enemy isn't full health, the enemy minion wave is building into a big mass, etc) so that you can get in and get out. The strength of Rengar is that you can be high mobility, high threat and are able to burst down unsuspecting champions. Don't put yourself in extended fights and don't wait around all day just waiting for a gank, otherwise you might find yourself behind in experience and gold, making you completely useless later in the game.