In several states if you are over 18 or 21 and not a felon you can concealed carry a handgun without a permit. If convicted of a felony you can not own a gun. In most other states you need to be over 21 and have a permit to concealed carry. Many states allow you to open carry without permits.
Rules about shooting people differ between states. Some states, if a criminal breaks into your house you can shoot without question, some you need to have the criminal attacking you and you unable to get away, while others (CA) you are pretty well screwed if you do and screwed if you don't. Laws also differ between states while outside of your home. Some you are legal to defend yourself on a public street if threatened, others you must try and run away, some (CA) bend over and take it.
Even if you are, say walking down the street and 5 convicted felons with illegally possessed handguns come up and state they are going to rob you, rape your wife and kill you both, you will still probably be forced to go through a long drawn out trial to clear your self. It is not the wild west where you could kill someone because he drew a gun on you and you just walk away without anything happening. Plus in many states the criminal's family can even sue you for defending yourself.
Better to be judged by 12 then carried by six.
Concealed carry permit (CCW) is similar to drivers license, you must go through training and pass a test, then the sheriffs office does a in depth background check.