Firearms are just tools, developed by humans and for humans through centuries to accomplish various tasks. These tasks may vary, but in my opinion firearms are as legitimate for civilian purposes as anything else, and according to statistics on accidents in many countries, firearms are less dangerous than automobiles.
Of various uses of firearms, I put the self-defense as most important for civilians. Self-defense is an essential human right, and no police, no matter how well-equipped and manned it is can't protect everyone at same time. Usually, when speaking on self-defense, handguns come to the mind first. However, handguns have limited effective range, limited stopping power (especially when used against wild and dangerous animals in the woods or the like), and also often hard to master. Long guns, such as shotguns or carbines, especially those of light weight and soft recoil, are much easier to fire accurately over any ranges beyond "an arm's length". Shotguns and handguns are discussed elsewhere on this site, so this section will mostly concentrate on rifles and carbines, suitable for self-defense (against humans and wild animals), home defense, as well as for general practice and recreational shooting (plinking). Dedicated sport / target and hunting weapons will be left aside, at least for a while, because either subject is too broad to be covered in available time.
image: Oleg Volk
image: Oleg Volk
And please always remember four rules of gun safety!
Rule # 1. Treat all guns as if they are loaded. Always check the gun before handling.
Rule # 2. Never let the muzzle of a gun point at anything you do not want to destroy or kill – always point the gun in safe direction.
Rule # 3. Keep your finger straight and off the trigger, unless you have aimed your gun and ready to shoot.
Rule # 4. Be absolutely sure of your target, and what is behind it.