Hello :3 I am Gold V Rengar main, my IGN is "Hi Im Luka xD" on EUNE server. I have been playing Rengar since end of season 5 and this is my first guide like ever xD
So lets get started!
First of all you might be asking yourself: "Why should I play Rengar?". Basicly if you like playing assassins like Kha'Zix, Akali etc. you will enjoy playing Rengar becaouse of his kit and his ability to one shot champions.

FIRST CLEAR / LV3 GANK

You should always start like this Krugs > Red Buff > Raptors. When I finish that clear i look if i can gank any lane (For example if you start on blue side of the map you would look if you can gank mid or bot, or atleast get enemies to flash), if you can't or don't want to gank just clear Wolfs > Blue Buff > Gromp and recall for items.

What to do?

As a Rengar you should look for kills and objectives and not so much for team fights becouse you want to get fed and end game as fast as you can. You should look for enemy players that are going alone on lane and try to kill them if you have ult and if they are not tanky and hard to kill (I'm not saying that you need ult to kill them but you usually want to ult to secure the kill).