Rengar is a maniac in top lane when he is in his level three stage he becomes a monster his q will tear through most top laners hp, still considered a Assassin he can take out opponents with ease in his early stages and level six to mid game.

Pro's

Great Early Laner
His passive helps him keep him alive
All his skills are useful for almost any situation
Great CS skills with his e and q

Con's

Flattens out late game (Like Talon)
Not as useful as other top laners
Takes some time getting used to.

Runes are stated above why these runes???
The runes give Rengar a tanky part of himself he needs as well as Pen so you can focus a little more on items.

21/9/0
You Can see that the Masteries make sense You may want to go 21/0/9 depending on the team Tanky or not, your goal is to focus the carry's while being able to leave the fight so please make the right decision in masteries.

Items are pretty simpel to understand Go with your gut feeling on this
Black Cleaver
Great item Gives Cooldown and Pen which is a must have one Rengar.
Give Rengar at least one or two tanky items such as Forzen Mallet or Trinity and then a GA or Zephyr.

Max your Q for high burst

Max your E first for Fighting at a distance early Game

Choose this yourself and then max the other second.

Why flash?
Flash is Useful because it gets you out of bad situations Use it if you want to live.

Why Ignite?
Great for more damage and with your masteries it gives more of a damage source.

Overall Rengar is a great champion, He has his moments where he can shine or where he can fall be careful when you tread Rengars path because its a difficult one so when you play rengar try to focus carries and live.

In fights USE YOUR ULT TO SNEAK BY THEM AND ENGAGE ON THE ADC OR MAGE IF YOU HAVE TO PULL OUT once the fight starts to happen if you have killed your target and have a decent amount of hp help out your teammates and if your low go get you Stacks up to 5 and heal yourself with you W then get back into the fight.