The Four Rules of Gun Safety
The foundational safety rules: treat every gun as loaded, never point at anything you won't destroy, keep your finger off the trigger until ready, and know your target and beyond.
The four universal rules of firearm safety are: (1) Treat every firearm as if it's loaded. (2) Never let the muzzle cover anything you're not willing to destroy. (3) Keep your finger off the trigger and outside the guard until your sights are on target and you've decided to fire. (4) Be sure of your target and what's behind it. They're intentionally redundant — it takes breaking more than one to cause a tragedy.
These rules apply everywhere, every time, regardless of whether you 'know' a gun is unloaded. Internalizing them is the first step for any new shooter and the constant habit of experienced ones. The overwhelming majority of negligent discharges trace back to violating one or more of them.