Early-stage app builder – what should I validate first?

Hi everyone, I’m in the very early stage of building an app and I’m intentionally keeping details private for now.

I’m mainly looking for general advice from people who’ve shipped apps: • What are the biggest mistakes at the idea/prototype stage? • What’s the first thing you’d validate before investing serious time or money? • Anything you wish you had done differently early on?

Appreciate any high-level guidance