This is the Number 1 rule of firearms safety. A firearm that is pointed away from everything will not be able to injure anyone, should it have an accidental discharge. Always treat every gun as though it was loaded. And always keep track of where your muzzle is pointed, even when the gun is unloaded.
Load your firearm only when you are ready to shoot. When you get done shooting, make sure to unload everything. Not just the chamber, be sure to unload the magazines also. Keep the action open on your gun when it is not in use. You should NEVER hand someone a gun with the action closed, nor should you accept a gun from someone without the action open, and then visually and physically check to make sure the gun is empty.
Do not rely on the safety of your weapon to prevent a round from firing. Safeties are mechanical items, and therefore are susceptible to failure. Do not have your finger anywhere near the trigger unless you are preparing to fire the gun. Most accidental discharges are caused by a trigger being activated with a finger placed on it when the shooter was not planning on shooting.
A bullet fired out of a gun can travel in excess of 1 mile. Make sure that you have a good, solid backstop. If shooting at game, make sure that if you miss (we all do from time to time), the bullet will not travel beyond your line of sight.
Every gun is designed to shoot a specific type of ammunition. There may be several names for the same round of ammunition. If you are unsure of what your gun shoots or what ammo you have, ask your local gunsmith. A firearm may chamber a different caliber, but firing the wrong ammunition could result in serious injury, dismemberment, or even death.
If you squeeze the trigger and the round does not fire, you may have a hang-fire. Revert to the First Rule; be aware of where your muzzle is pointed! Keep your face away from the breech of your gun, put the safety on, and carefully open the action. Take care to properly dispose of the round. Remember, just because the round did not fire, it is still a loaded round of ammunition, so treat your gun as a loaded weapon.
You risk great damage to your eyes and / or ears by not wearing proper protection. A stray fragment from your ammunition could end up in your eye, there could be a mis-loaded round that causes extra-high pressure and creates a disaster; anything can happen. The noise created by firing a firearm will damage your hearing after only a few rounds, and ear plugs only cost a few cents. What are your eyes and ears worth?
Before firing your gun, open the action, and after making sure it is unloaded, look down the barrel to ensure that you have not collected any debris in the barrel. A small amount of dirt, snow, grease or even a stray spider's nest could increase the pressure in your barrel to the point where the gun could explode. Never try to shoot an obstruction out of your barrel, you most certainly will be injured, or at least ruin your firearm.
Your firearm was designed by engineers to function a certain way. Modifying or dis-engaging any safeties of portions of your firearm will only serve to endanger you and possibly ruin your firearm.
Guns from different manufacturers are very different. The individual characteristics of each gun should be learned inside and out before you chamber the first round. Read the owner's manual before ever shooting any firearm, no matter who made it.