Typically a well-protected home will have on hand several guns each of which excels for the different scenarios a firearm may be needed for. At a minimum you would best be served by owning a handgun, a shotgun, and a rifle. Add to that a rifle that fires the .22lr round for basic survival.
For rifles, perhaps the best to start with is the .22lr Plenty of inexpensive ammo (a few cents per round), easy to shoot with little kick. Great starter rifle to train with that is also is oft considered the best survival rifle of all. Handguns firing .22lr are also excellent for training.
Shotgun: 12-gauge is best for home defense, for smaller people a 20-gauge will do.
Handgun: for self defense anything with a "40" such as .45acp or 40S&W is best. But 9mm will do and is very common - for smaller people the 9mm generally works well.
Go with common calibers so that should the SHTF you will be more likely to find ammo. Some very common ammo that many households and stores have on-hand include:
9mm, .45acp, 12-guage, .22lr, .223, 30-30, 7.62x39, .308
For a man, I might recommend acquiring the following 5 firearms:
.22lr rifle, 12-guage shotgun, .45acp handgun, choose one rifle: (.223, 30-30, 7.62x39), and a .308
for a woman of smaller build these four firearms would set you up well:
.22lr rifle, 20-gauge shotgun, 9mm handgun, .223 rifle
Be sure to have plenty of ammo and at least half a dozen magazines for each firearm.