There are 2 major schools of thought, both similar.
Jeff Cooper is credited with these four rules.
1. All guns are always loaded.
—Even if they are not, treat them as if they are.
2. Never let the muzzle cover anything you are not willing to destroy.
—See rule #1
3. Keep your finger off the trigger until your sights are on the target.
—Keep your finger along the frame of the firearm above the trigger guard.
4. Identify your target, and what is behind it.
—Never shoot at anything that you have not positively identified.
The fundamental NRA rules for safe gun handling are:
1. ALWAYS keep the gun pointed in a safe direction.
2. ALWAYS keep your finger off the trigger until ready to shoot.
3. ALWAYS keep the gun unloaded until ready to use.