How you carry a gun is important. Generally speaking the idea is to find one carry that works for you and use that pretty much exclusively. During times of stress your body tends to revert to training and you could end up reaching for a gun that isn’t there because it is someplace else on your body.
First; shoulder holsters. These come in three basic types, vertical, horizontal and upside-down. The vertical shoulder holsters carry the gun muzzle down and the butt forward. If you have a good harness they are reasonably comfortable. If you carry spare rounds on the opposite side they balance fairly well.
Horizontal shoulder holsters carry the gun horizontal with the muzzle pointed straight behind you. Obviously if you have a very large weapon or are a very small person this will be an issue.
Upside-down holsters have been around a long time, mostly for smallish revolvers. They can be very concealable but you need to make sure the retention is solid or you can end up with your gun going bouncy-bounce down the street.
If you have short arms, a barrel chest, big boobs or are just plain fat shoulder holsters can be slow and awkward and obvious. If you spend a lot of time in a vehicle or otherwise sitting down they can be very handy. You must remember that the way they are designed they are easier for the bad guy to grab than for you to grab so you want to be very reluctant to wrestle with someone while wearing one.
Ankle holsters are also very concealable but are somewhat slow to access. They can be uncomfortable to wear after a while. You can carry a weapon with one so it is reachable with either hand. In my experience the ones with the suspender belt above the calf are more workable than the ankle-band only variety.
Pocket holsters can be very concealable and are actually fairly fast to access as long as you are standing up and are not wearing really tight pants. You really want to use a pocket holster if you do this as it keeps the gun from rotating in your pocket and keeps the gun from printing thru too obviously.
Small of the back (SOB) holsters a fairly concealable but I confess I don’t like them. They are generally slow and awkward to use. Also if you fall onto your back or are hit hard in the back the gun slamming into your spine can definitely smart. In addition your cover garment may snag on it and you may be unaware of it. The whole idea of concealed is to be concealed.
Belt mounted cross-draw or appendix holsters can be very concealable and fast, but while sitting down can be difficult to access depending on your body type. They do make special holsters just for sitting down, the TSA had a good one designed for Air Marshalls.
IWB holsters (inside the waist band) are relatively good on concealment, especially with a fairly flat semi-auto. If you have a good body (more hips than gut) they can work well but can be uncomfortable. A slightly loose belt to hold the holster and suspenders to hold up your pants might do the job for you.
Clip-on holsters. I admit I am prejudiced. I hate these things. I am always afraid the gun will wander around, twist or even fall. I just plain don’t like them. They look good for TV detectives however.
Strong-side hip holsters are a good way to go for most people. You can place the holster just behind the point of the hip, get good access and good concealment. With the muzzle raked slightly back the weapon is fairly easy to grab with the strong hand, but very difficult to access with the weak hand. It is reasonably quick to get to and isn’t obvious that you are reaching for a gun. You could be reaching for your wallet.
Off-body carry, i. e. purse holsters. I don’t like them. Unless you carry this thing with you constantly you don’t really have your gun with you. Also the carry object could easily be the target of a serious theft attempt. Maybe I am just a radical but I think if you are going to carry a gun you should CARRY a gun.
Fanny packs. These do the job but unless you are actually out jogging they scream I HAVE A GUN.
Leather vs synthetic. I confess to being a traditionalist. I like blue steel and leather. That being said stainless steel and Kydex do have their place. I don’t like ballistic nylon holsters just because I think they are ugly and cheap looking. They certainly work. The harder synthetics like Kydex can dig into you a bit so you might have to work harder to find a spot to carry them.
Don’t neglect spare ammunition. It is statistically unlikely that you will have to reload your weapon in a private citizen self-defense shooting, but it is not impossible. After all, if you have been involved in a shooting the odds have already gone against you. It is a very good idea to have at least one reload with you. If you carry a revolver a “speed strip” with the correct rounds dropped into a pocket is adequate. A spare magazine for a semi-auto is smart. Ammo dump pouches are so old and semi-obsolete that a person observing them on you might not know what they are. Don’t have a spare magazine just floating around loose in your pocket, it is too easy for it to get crudded up. If you don’t want to wear it on your belt they make neat little magazine cases. They will slow you down but keep your reload usable.
I can honestly predict you will spend some time and some money getting it right. It isn’t money wasted. By the way, do not STORE a gun in a leather holster or gun rug. Leather is hydroscopic, it attracts moisture. Modern leather is acid tanned. Leaving a gun in leather except in an extremely low moisture environment is like giving your gun a constant low-grade acid bath. It will ruin the finish of a blued gun.