John Farnam's "The Farnam Method of Defensive Handgunning" is, in my opinion, the best book available on the proper techniques of defensive handgun usage and carry. Read it, ponder, and read it again.
Then, take a class with a good instructor. Of all the classes I've taken, Clint Smith and John Farnam are, in my opinion, the two best instructors out there. Both have "seen the elephant" and both have the rare skill of being able to teach clearly and concisely.
Then, practice what you've learned in the class. You won't leave the class being a steely-eyed dealer of death. You have to practice the techniques you've learned for a couple thousand repetitions over a period of time before they become grooved and available at a subconscious level.
After good instruction and weekly practice, the average person can become reasonably competent in a couple of months. Take another class six months down the road and continue to practice and build your skills and you will be better than 95% of the people out there within a year. The 5% above you, however, are so good that it's frightening... be thankful that most of them are the good guys.
Note that even though you'll have a good handle on handgun manipulation and shooting technique, there is more to surviving a gunfight than this. There's also the tactical mindset... what to do and when and how to do it. Clint Smith teaches about the survival ladder, four things listed in order of priority that will give you the best chance of prevailing. They are: situational awareness (knowing what is going on around you so you can avoid trouble if at all possible or get yourself out of a developing situation without requiring the use of force), tactical proficiency (knowing how to position yourself against your opponent to maximize your advantages and his disadvantages), skill with the available weapons, and choice of weapon. You really can't get this from shooting IPSC or IPDA.
You'll find that 'gunmanship' is as much a martial art as any style of karate, and while a yellow belt has a step up over people with no formal instructional background, to reach the 'black belt' level will take years of study in different areas (unarmed self-defense, weapons disarming and retention techniques, defensive and offensive tactics, etc.).
How good you become depends on how much time, effort, and money you're willing to invest... with the onus being on time and effort.