1. Always treat a gun as if it were loaded- It's simple common sense, but unfortunately not always followed. While guns are ultimately tools, they are also deadly. And this rule doesn't mean treat the gun differently after you've checked to see it's unloaded. ALWAYS, act as if the gun is loaded. So, loaded or not, you should never find yourself pointing a gun at someone who is not an immediate threat to your life; whom you are willing and preparing to shoot. The importance of this rule cannot be overstated, and if followed, will be your foundation for good gun safety for years to come.
2. Never point your weapon at something you are not ready and willing to destroy- This one really goes hand in hand with the first rule. If you aren't willing to shoot something, intentionally or not, don't point your gun at it. This goes for friends, the family pet, and anything else that you wouldn't want to harm. This doesn't mean you can point the end towards something if it's unloaded and you don't have your finger on the trigger, it means NEVER point the end towards something important. Drawing a hard line in the sand and strictly adhering to this rule will greatly reduce the chance of gun related injury; including especially, accidental misfires or discharges.
3. Don't place your finger on the trigger until the target is in your sights and you're ready to shoot- A target is anything you are deliberately aiming at. Poor adherence to this rule is how people shoot themselves in the foot while drawing their gun. In a dire situation you may need to move quickly, but you should never rush and place your finger on the trigger before you're aiming down the sites and ready to fire. Until that point, many choose to rest their finger firmly along the slide of the gun. This makes it easy to shift into a firing position if need be, but lowers the odds of accidentally firing the weapon because you slipped.
4. Always be sure of your target and what's behind it- More common sense that is easy to forget or ignore. Bullets have little trouble passing through objects and continuing on into other unintentional objects. I myself grew up shooting out on the farm and regardless of what kind of target we were shooting at, we always made sure there was an embankment behind it. This rule could also apply in a situation where you are attacked. You may be dead accurate and able to hit your attacker, but in a crowded mall what might you also unintentionally hit after the attacker?
Always follow the above 4 rules of gun safety.
Make sure you know the mechanical aspects of your gun, as well as the way it handles.
Make sure you use the correct ammunition.
Don't rely on your gun's safety.
Always be aware of your surroundings, this includes any spent casing or other debris that may be found on the floor.
If your gun does not fire after pulling the trigger, wait a few seconds before unloading the gun and racking the slide. Make sure to do all this with the muzzle pointing downrange.
Always keep your barrel pointed downrange.
Always were eye and ear protection.
Respect your fellow range goers.
Fully familiarize yourself with and follow all rules of the gun range.
Practice hard, shoot well, and have fun!