I've taken the Tactical Pistol Instructor course from Sigarms Academy, and I've also taken all of their CCW courses as well as a pile of certs from other schools. This is not just my hobby, it's my job now - at least one of them anyhow.
My first advice would be to take some training if you haven't already from ANY reputable school, such as Thunder Ranch, Fronsite, Gunsite, Sigarms, ANYWHERE. Just take some training, regardless if you have L/E or military experience, as CCW stuff is much different from what you would have learned previously.
Secondly, your choice of caliber and firearm should be based on comfort, your hand size, what type of medium you may have to shoot through, such as glass windows in your car, your attackers clothing (heavier/leather in colder climates) etc etc. There are a lot of factors, but you really can't go wrong with ANY firearm, so long as you have good basic skills in getting it out and using it, in addition to discretion as to when this should occur.
My best advice would be go with what you're already familiar with, and be sure to carry at least ONE extra magazine. Sigs, Glocks, 1911's, HK's...every major brand out there will have a weapon that can serve you well. On my range, the Sigs and 1911's outlast everything, I've yet to have a Glock or HK make it past 40,000 rounds, and have pictures on my server of Glocks that have come apart at the serial numbers in under 5000 rounds. Just be aware any gun can break at an unpredictable time, so having a backup system for CCW and good access to armourers and parts is important to consider.
Sigarms gets feedback from Naval Special Warfare, the FBI, the USSS, you get the picture, as the academy trains all of these units and then some. The one common thread I've noticed from the feedback these folks give is that people drain ammunition at much greater rates when the **** hits the fan, and many CCW holders in the USA have made the error of not having at least one extra magazine or speedloader. This is probably the most critical and common mistake.
So, get a good reliable pistol, train with it and practice your CCW draw, malfunction drills, all of the basic stuff, and also get good accessories, such as a VERY good holster and mag pouch. That brings up the point of how you plan to carry your weapon - Inside the pants, high ride, shoulder rig, small of back, fanny back...they all offer something different and require different drawing techniques so you minimize the chance of you blowing off fingers of your non dominant hand if you have to engage somebody who rushes or grabs you.
You'll want to have yourself trained to be very consistent and fast with your gross motor skills, as every "expert" out there agrees that you'll lose your fine motor skills, be affected by your bodies startle response, get auditory exclusion, and have the lens of your eye flatten out, IF and when you have to engage somebody who is a threat to you.
Also, if you think you may have to go into a higher threat area, consider getting yourself a good back up weapon, such as a very small auto or revolver, and practice as much with it as your primary weapon.
Practice practice practice. The more you draw up, press that trigger, and verticle track your target , the more likely you'll be to do it right when you need to.
