Since everyone else is giving the same advice, I'll give you a different kind. I'm a game away from my Diamond promotional series, and I've got there almost entirely by playing Rengar. So, here's a few tips on what you actually want to hear.
Pre-6, Rengar can't gank mid. It is extremely unlikely that you will ever actually get anything from visiting that lane. In recent memory, I can recall exactly one game where I got that gank off. I had just finished krug-raptor-red, and I was roaming by and I saw Yasuo waveclearing with his dashy dashy thing, and he didn't have minions to dash back to. He was literally at our turret, and my laner was playing some utility mage that I can't recall. If you somehow get six stacks before you're six, the leap range is increased far enough to be able to dash to the center of the lane. In most situations, ignore mid lane pre-6. You can't gank it.
As for top and bottom lane, my first advice is going to be in the top lane. On your first clear, don't back. Top lane is usually a volatile match up in which both laners are constantly trading, and thus, <75% hp. Most top laners at every elo I've seen will go for the level two all in. Right about the time you go from buff - camp - buff, or camp - buff - buff, they'll have hit the all in and they'll both be out of pots and/or struggling to sustain back up. This also means that one of them has a lane advantage. Either your laner is overextending and you'll be able to sneak into a bush, or they're overextending and you can walk straight up and gank them. In the event that your laner is losing, do not get five stacks. You want the initial slow from your bola. The initial slow will often force the flash, or allow for more follow up cc from your laner. After this, use your bola so that it does not miss. In lower elos or against careless laners, you can usually do this at least one more time. On a closing note for top lane, don't be afraid to turn on the jungler if they come. This can easily be a double kill, multiple ss burns, or the start to a bad game. Be careful. As for bottom lane, it's literally the same advice for top lane with a few differences. Bottom lane rarely goes for the all in trade at the early levels, so you should already have a kill or two from the top lane if things are going well. Sneak into a bush if your laners are extending. Make sure it's not warded, as this sets you up for a roam from their mid/jungler. In the event that their laners are extending, wrap around their tri-bush. Be careful not to get turned on, and make sure you ping your teammates to engage.
I assume you know how to do the rest.
