Years ago, people had to learn about defensive shooting on their own and a lot of them didn't survive the first lesson. And the same thing still happens today, but it doesn't have to.
Any good defensive shooting school should be a life-changing experience for the student. Qualified instructors can teach the new shooter the kind of shortcuts that it would take him years, if ever, to learn on his own. And a good instructor can quickly analyze and correct the bad habits that experienced shooters have picked up over the years. In short, a good defensive school can put a person on the fast track to having a successful defensive plan.
But, I caution you not to just sign up for any old defensive shooting school just because it is close to where you live or it is cheap. Truly qualified instructors don't work for free and the worthwhile school has facilities to build and maintain. All of that costs money. Just like buying good quality guns, ammo, and gear, the serious defensive shooter will budget good training into his program as well. Top quality shooting schools are expensive, so budget your money and save for your training.
I can personally recommend the Gunsite Academy, Clint Smith's Thunder Ranch, and the Sig Academy. I have attended these schools and can vouch for the high quality of training that they offer. I'm sure that there are other good training facilities out there, but I will only vouch for those that I have personal knowledge about. If a student is looking at another school, he should check them out every way possible before laying down his hard-earned money. It's like anything else, you are going to get what you pay for. If a deal looks too good to be true, it probably is.
I don't buy the old story about certain people being natural-born good shots. All of the good shots that I have known practiced a lot and trained a lot. And they weren't above taking some good suggestions from professional instructors. If you carry a defensive handgun, get professional training. You owe it to yourself and to your family.