1) Choose a gun that fits you well and that you shoot well. Make sure it suits your needs and/or wants for a carry gun.
2) Find a good holster for said gun. Don't skimp here, as it's the biggest thing you can do to carry comfortably and conceal well, even with a full sized gun.
3) Get a good gun belt that can handle your rig. This, too, is a key step to not skimp on. A great holster in a wimpy dress belt won't do.
Obviously, size and weight come into play, but a good holster and belt can mitigate these factors. My carry gun has changed significantly as my knowledge, experience, belt, and holsters have improved.
In general, the grip and the width of the gun are two factors that can adversely affect concealment, but a good holster/belt combo helps with these.
For belt suggestions, if you want leather,
the Belt Man makes good leather belts. If you want nylon, there are several good belt makers, but a lot of people like the
Ares Gear belts.