Take the training, try as many different pistols as you can. Find the ones that YOU shoot well. (grip angle, grip design, sight design, overall geometry, etc will all come into play in how YOU handle specific guns.)
SIGarms is a good school, but you might want to attend at least one independent course where you can use guns that are not all one brand (or make the effort to go to a large member shoot/rental center and try different things)
Try to take a course where you can try several carry methods. See what you prefer and what will fit your lifestyle. The Members section keeps bringing up the "Holster Party" idea to try all kinds of designs, but location/time/etc all seem to never come together.
Another way to try a gun is to ask. Most shooters I know will be happy to allow someone to try a gun out. It is considered good manners to bring ammo if you intend to give it a good workout, but that varies. I know I don't like some brands of ammo put into certain guns and would rather supply my own than not.
Try to stick with common ammo calibers (.38 special, .357 Magnum, 9x19mm, .40 S&W, .45 ACP) More esoteric calibers (.357 SIG, .45 GAP, etc) may offer something to you, but ammo availability for practice and such is not nearly as common.
Since the S&W .500 and S&W .460 came out, I have to special order any .480 Ruger I want to load in my Bear gun because there just isn't any demand at all. I suspect in a few years, it will be handload only for that caliber. You don't want to get caught in that kind of situation for a carry pistol.
Select a combination you've decided might work. Try it. Be prepared to reject it. Unfortunately, you will have a drawer of holsters as you discover what works and what doesn't. Hopefully you won't have to hunt for a gun you like - that can get expensive.
While cheaper 'plastic' holsters might seem a good way to 'try' a means of carry, rarely do they do the position justice. A top quality holster can make a location feel perfect where another slightly different design might be completely wrong. I still prefer leather for long term carry over any of the Kydex designs just because leather tends to better mold into position and make less noise when you move. But good leather is NOT cheap.
Be aware that some holster features don't always work. For example, that flap that lies between the upper end of the gun and your body might sound like a great idea until you find out that sometimes the flap moves, covers the holster opening, can pinch, etc.
A holster that NEEDS a strap is usually an indication that the holster isn't holding the gun right.
Be prepared to discover that you have 2 or even 3 different setups that you prefer depending on situations.
Make the time to get some practice and range time. You can practice drawing and such at home with a VERIFIED empty gun, but you need to get to the range to keep marksmanship skills sharp. Does no good to carry if you can't use it. This year has been tough for me, but I've made the effort to go at least once a month and empty my current magazines on the range. Its only 45 rounds, but it's 45 rounds most people do not ever do. (that's 3 10 round mags for the M&P and 3 loads of 5 rounds on the .38 snubbie)
In any case, feel free to decide for yourself regarding anything. Just because your choice is not what every gun 'expert' recommends, if it means you are better able to carry, then it is a good choice for you.
You'll hear a lot that "A gun should be comforting, not comfortable." That's BS. If it isn't comfortable, you won't wear it. Make the effort to find the gun and carry system that makes it work for your routine.