League of Legends (LoL) Question: New Rengar tips

That's a pretty broad question. For a broad answer, I recommend this guide. The guide has very detailed information about matchups, item selection, and proper use of Rengar's abilities. It's from Season 8's pre-season, so all of the information is still sound (except the guide's creator would probably now suggest getting the Duskblade instead of the Ghostblade). For some quick tips, here's what I've got:

Rengar does well in team comps that have a tanky Top laner and plenty of Crowd Control, or Dive comp teams that can engage from a distance with him.

Rengar plays well as an assassin, but can still put out decent damage when building tank items.

Your main stats are Attack Damage, Lethality and Cooldown reduction, followed by defensive stats (HP and either armor or MR), making the Duskblade, Black Cleaver, and Death's Dance very popular items for Rengar Players.

I like building a Tiamat when I play Rengar, and once upon a time, it was common to build a sheen/triforce, but unless I'm mistaken that's fallen out of favor.

For Runes, I recommend Domination Tree with Electrocute or Predator, Sudden Impact, (any of the next tier), and either Ravenous or Relentless Hunter. Any of the secondary trees would work fine, depending on your preference. If you're just looking for a place to start, Precision with Bloodline: Tenacity and Triumph is safe and helpful.

If you wait until next patch, he's basically old Rengar again.