I think the first question you need to ask yourself is the purpose of the firearm. Are you going to carry daily? Is it going in the night stand for home defense? Think about what purpose it will serve. I would then go to a gun range that has guns you can rent. Explain to the gun range personnel that you need to shoot a few different types of guns to see what you like. I would shoot a polymer framed semi auto, a metal framed semi auto, and a revolver. I believe that the 9mm and the 38 special rounds are good "starter" rounds. I hope this helps.
This is an intricate question.
It all depends on why you want or need the gun?
For self defense, the ideal gun is a 45ACP pistol. You don't need anything more potent, and anything more potent shoots more slowly and has a higher risk of collateral damage.
If you cannot take the recoil of a 45ACP then you can downgrade to a 9x19 which is a bit lighter.
If you cannot handle the complexity of a semi auto pistol, then a 38 special revolver is the ticket.
I would not recommend a mouse gun of a smaller caliber.
Nor would I recommend a heavy cannon of a larger or longer caliber/cartridge.
You need to go to an indoor shooting range which rents guns and which has instructors and sample several.
Once you have settled on a choice, then you need to take many many many classes in shooting.
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