Whilst some street samurais prefer the ancient katana or more esoteric monofilament weaponry, there's nothing that the average solo finds more trustworthy than their gun. You know what you're doing with a gun; you point it at someone and pull the trigger, and their head explodes into a fine red mist. Satisfying.
In game terms, guns fall into categories based off the skill used to operate them. Most handguns and revolvers fall under the pistol skill, while larger weapons fall under the rifle skill. In either of these groups are weapons that can be switched to fire in automatic or burst fire mode, which uses the submachinegun skill. A rule of thumb is that if the weapon is described as pistol gripped and held in one hand, it'll use the pistol skill (this includes things like sawn-off shotguns).
Many guns will have a minimum strength requirement, which increases with the calibre of the weapon. If you're unable to match this requirement you'll find your shots go wide due to recoil, and you may even drop the weapon. Due to their two handed nature rifles have a slightly lower strength requirement compared to a pistol of the same calibre, though modifications can be made to pistols to alleviate this. You'll also be penalised slightly less if you're using your pistol with both hands (simply by having your off hand free).
Using a gun
First up, you'll need to load your weapon. The majority of guns in the city of New Carthage are magazine fed. You'll need to source an appropriate mag for your weapon; Arms Unlimited not only sells guns, but also boasts a bulk ammo machine to buy ammunition from. Alternatively, players with the Ammo Loading advantage and the appropriate hardware will be able to make more advanced ammunition.
When you have the magazine, loading the gun is straightforward:
load <gun> with [first, second, etc] <magazine>
To retrieve the magazine you can:
There's no need to otherwise ready the weapon in most cases, the game is considerate enough to assume your character knows what they're doing with the loaded firearm they're waving around and wouldn't negiligently discharge a round. On which note, you can:
...to negiligently discharge a round.
You might be better off aiming the gun at your target first, then firing. You'll not only shoot at them but initiate combat at the same time. You can aim the gun at yourself and fire if you wish:
aim <gun> at <player>
Revolvers and other guns
The above information about loading your gun will get you by with most of the weapons you'll encounter, but it's worth noting that revolvers, having a cylinder, operate slightly differently. You will need to source individual bullets, rather than a magazine, and load each chamber of the cylinder individually. Happily, this allows you to play Russian Roulette:
open <revolver> load <revolver> with <bullet> close <revolver> spin <revolver> aim <revolver> at me fire <revolver>