Roughly what I type when a new ST asks for advice:
I run games mostly by improv, though I prep more for games with strong continuity like Vampire which usually stays in one city unlike games with the PCs travelling to new adventures. This means the prep work is more frontloaded at the start of the chronicle.
I generally have a rough idea for an opening scene and ways it might go, though don't be surprised if the players do something else entirely - give yourself a moment to think when you're surprised.
I prepare most for things I'm not confident about improvising, like names for NPCs. It's pretty obvious who I was expecting the PCs to talk to if one is introduced as "Alessandra Chen, of the 67s" and the next as "uh... Bob the Gangrel."
It can help to have some ideas noted for 'standard' scenes like hunting, considering how the different PCs approach it. What's a good name for a club on the Rack? Something else I like to have a few ideas for in advance.
I very rarely use building maps as I don't run much tactical combat, though I've found Google Maps getting more use for cities while playing online - normally I'd handwave details like where subways actually go, but some players like to have more grounding.
With so many NPCs I also find it helpful to have a visual reference. I mostly use photographs to keep it consistent.
Good luck with the game!
