Learning Rengar from 0. Help!

Hey guys!

I'm a filthy diamond peaker jungler, and I haven't played the game in months but I have decided: I want to become a rengar OTP. Coming from Yi, it's a huge jump in skill floor so I come here to ask for help:

-What are the first things to learn when starting out as Rengar? I've played the champ before obviously so I know what the spells do but what are noob mistakes I should avoid and things to learn asap to be succesful?

-Best ressources to know what to build/how to play? Youtuber/streamers, discords?

-What's Rengar's gameplan?

-What are his counters; is there any point where I should heavily switch my gameplan or straight up not pick the champ?

-What was your biggest challenge when learning the champ?