I was brought up in a non-gun household in New Jersey. My father was pretty much a no gun kind of guy. (Despite the fact that his grandfather was a Paris Expo award winning shooter). So we didn't have guns.
Years later, I decided that I wanted to shoot, and asked my gunner Brother In Law, Mike Bravo, to help me learn and decide what gun to shoot. When I asked him "What gun should I get?" His answer was perplexing to me, he told me to decide on ammo, and then find a gun I liked that shot that ammo. It seemed backwards to me then, but now I understand what he meant. Let me help you out by telling my story, and hopefully saving you a ton of cash and grief.
My first day shooting was after a lengthy session on gun safety in Mike Bravo's den. This was a good thing: (a) It helped me to pass hunters safety, since I gained a thorough (and I mean thorough) understanding of gun safety and gun anatomy, (b) I learned all about different kinds of action, and how to work them, (C.) and I had learned about eye dominance. Mike Bravo took me down to a gravel pit to shoot several different guns, from the 22 rimfire up to a 12 gauge shotgun. I learned a lot that day, about what I liked and what I didn't like. I would heartily advise the new shooter to have someone take them to the range and shoot a variety of guns before they buy a gun.
I had decided that as a first-come shotgun made the most sense, since there wasn't anything that I couldn't hunt with it (using different loads or slugs), it seemed to be an all-around good choice. I consider myself a kind of traditionalist so I felt that it over under Shotgun was really what I wanted. When I went to the gun store to look at guns, I have found one that seemed to fit my price range and anesthetic wants and needs. As luck had it the gunsmith was at the sales counter that day. And upon shouldering that shotgun, in two seconds told me that it was not begun for me. I didn't realize at that point that guns needed to fit you, and not the other way around. He was kind and gentle, and really help me understand a lot. He probably could've sold me that gun and I would've been okay with it, and he would've made a sale. But he probably would've ended up selling one gun to me ever. He took around and had me shoulder many different guns. Ones that were within my price range, ones that were cheaper than I would've ever paid, and ones that were way more than I could've ever dreamed of spending.
I had gone in with the intention of buying a 20 gauge over under shotgun, and I left with a 16 gauge side-by-side. I was told that the 16 gauge was coming back and that my concern about ammo availability was unfounded. That gun fit me perfectly and when I went to the range whether I could not believe how well I shot it. After I run through the few boxes that I bought with the gun I went back to go buy more ammunition. Hunting season was well underway and 16 gauge ammunition that the store had purchased for the season was now gone. Finding more proved to be difficult. Every store I went into, from box stores to megamart's, small gun shops, had no ammo, or really weird in ammo. If I wanted nine pellet 16 gauge I was in luck or number two goose shot also in luck... a regular hunting load was not to be found. Don't even get me started on buckshot and slugs...
One day I stumbled upon a cache of number six shot, and I bought the whole case. That was the last that I found for the entire season. I finally understood what my brother-in-law was talking about when he suggested that I look at ammunition first, then fit.
Here is my short list (based on my own personal preferences) for first Time gun buyer ammunition; for shotgun 20 gauge. it is widely available, you can buy cheap or expensive ammo, you can buy slugs, buckshot, and even some weirdo loads. It doesn't kick badly, and it is very sweet to shoot.
For a rifle, 308. It is readily available, shoots well, and will take anything inside North America.
For a handgun, I would probably recommend For a revolver, 38 special, and for an autoloader 9 mm. Again they shoot well, they are readily available, and are affordable.
Being able to find ammunition is really important. Otherwise you really just have a very expensive club. I know there are a lot of people (myself included) that are enamored of the 16gauge. It is the perfect blend of the sweet shooting 20 gauge and a very powerful 12gauge. Ammunition is a gigantic pain in the ass to find.
The ability to walk into just about any gun shop and find something to feed your gun cannot be understated. Going with the standard gauge ammunition for your first gun is very important, you should be spending time at the range shooting, not time and gun shops finding ammunition.