Taining comes at many different levels for different purposes. I think the OP is referring to a novice who wants a gun, but who knows little about guns. The following is for that type of person.
The first thing to know is basic safety rules. Many assume safety is a simple task, but it really is a little more complicated than it appears. Without some sort of training to make one aware of the various safety factors, one most likely will not apply what most experienced and trained gun owners practice. I have seen people very knowledgeable about guns who were not aware some safety factors -- These people perform some the most obvious violations there are. For example, one extremely knowledgeable person in my gun club would invariably pick up a gun and put his finger on the trigger. He never left his finger off the trigger, almost always didn't clear the gun. And he had poor muzzle control. We finally got him straightened out so now he is safe. So you see some sort of safety training is necessary.
Part of that training will be detailed descriptions of various kinds of guns: revolvers, semi-automatics, single action, double action, mechanical safeties, rifles, shotguns; various ammuntion, how ammunition works, etc etc, etc.
If the interest is in handguns, the best beginning course is the NRA Basic Pistol Course, or Rifle Course, or Shotgun Course. Personal and Home Firearm Safety.
Once you have the basics, then and only then should you consider target shooting, hunting, self defense, etc.
Cogito, ergo armatum sum