Just got a PS5, what should I play first?

Play Astro's Playroom before anything else, it's already installed and it's basically a tour of what the controller can do. Takes a few hours. After that depends what you like, but Astro Bot, Ratchet & Clank Rift Apart and Spider-Man 2 are the ones that show the console off. If you want something longer, God of War Ragnarok. Play 2018 God of War first if you haven't, it's on PS5 too.