Level one should be very easy for rengar. if you can hit level 2 before your enemy you can get first blood easily when you get e and q.

You wanna buy another doran blade and work on your tiamat and esrly boots.

Follow the item build when you have boots hydra yomno infity edge last whisper maw of malmortius/botrk depending on enemy team. sell your boots and get trinity force you will get almost the same speed and more if you have 3rd dragon buff you will also get movement speed from your necklace stacks and yomon active.

Some of the neat tricks you can do with rengar:

1) If you e mid air to your opponent it will be so hard to see and dodge even you cant notice it.

2) Using your ulti followed by teleport will allow you to reach lane invisible tricking your opponent into thinking that you cancelled your tp. But make your move quick and jump on them as they will notice the exclamatory mark warning them that you are about to jump on them

3) Always fight in a bush as your jumps will give your enemy a less time to react and its a great gap closer. And easily a stick on tool if you are facing a ranged enemy.